diff --git a/cmd/archivmail/main.go b/cmd/archivmail/main.go
index 8a2f49c..3b98293 100644
--- a/cmd/archivmail/main.go
+++ b/cmd/archivmail/main.go
@@ -104,7 +104,7 @@ func main() {
 	}
 	defer mailStore.Close()
 
-	// Index
+	// Index — per-tenant index manager (PROJ-21 Phase 4)
 	indexBackend := cfg.Index.Backend
 	if indexBackend == "" {
 		indexBackend = "xapian"
@@ -113,21 +113,24 @@ func main() {
 	if batchSize <= 0 {
 		batchSize = 100
 	}
-	idx, err := index.New(cfg.Index.Path, batchSize, indexBackend)
+	idxMgr, err := index.NewTenantIndexManager(cfg.Index.Path, batchSize, indexBackend)
 	if err != nil {
-		logger.Error("index init failed", "err", err)
+		logger.Error("index manager init failed", "err", err)
 		os.Exit(1)
 	}
-	defer idx.Close()
+	defer idxMgr.Close()
 
-	// Async index worker
+	// Global index reference for backward compatibility (IMAP importer, etc.)
+	idx := idxMgr.Global()
+
+	// Async index worker — tenant-aware (routes docs to correct per-tenant index)
 	asyncQueueSize := cfg.Index.AsyncQueueSize
 	if asyncQueueSize <= 0 {
 		asyncQueueSize = 1000
 	}
-	worker := index.NewWorker(idx, asyncQueueSize, logger)
-	worker.Start()
-	defer worker.Stop()
+	tenantWorker := index.NewTenantWorker(idxMgr, asyncQueueSize, logger)
+	tenantWorker.Start()
+	defer tenantWorker.Stop()
 
 	// User store
 	users, err := userstore.New(cfg.Database.DSN())
@@ -186,6 +189,7 @@ func main() {
 	}
 	defer tenantSt.Close()
 	srv.SetTenants(tenantSt)
+	srv.SetIndexManager(idxMgr)
 
 	// Start SMTP daemon with index worker integration
 	if cfg.SMTP.Bind == "" {
@@ -193,7 +197,9 @@ func main() {
 	}
 	smtpDaemon := smtpd.New(cfg.SMTP, mailStore, logger)
 	smtpDaemon.SetIndexCallback(func(raw []byte, id string) {
-		submitToWorker(worker, mailStore, raw, id, logger)
+		// Look up the tenant_id for this email from DB metadata.
+		tenantID, _ := mailStore.GetTenantForMail(context.Background(), id)
+		submitToWorker(tenantWorker, mailStore, raw, id, tenantID, logger)
 	})
 	// Wire tenant routing into SMTP daemon
 	if cfg.SMTP.TenantRouting == "domain" {
@@ -220,6 +226,16 @@ func main() {
 	// Wire LDAP config store into API server
 	srv.SetLDAP(ldapSt)
 
+	// PROJ-23: Per-tenant LDAP config store
+	tenantLdapSt, err := ldapcfg.NewTenantStore(cfg.Database.DSN(), aesKey)
+	if err != nil {
+		logger.Error("tenant ldap store init failed", "err", err)
+		os.Exit(1)
+	}
+	defer tenantLdapSt.Close()
+	srv.SetTenantLDAP(tenantLdapSt)
+	authMgr.SetTenantLDAP(tenantLdapSt, tenantSt)
+
 	// Wire SMTP daemon into API server for status endpoint
 	srv.SetSMTPDaemon(smtpDaemon)
 
@@ -247,7 +263,7 @@ func main() {
 	srv.SetPop3(pop3St, pop3Imp)
 
 	// Backfill in background: migrate existing files into DB metadata + re-index
-	go runBackfill(context.Background(), mailStore, idx, worker, logger)
+	go runBackfill(context.Background(), mailStore, idx, tenantWorker, logger)
 
 	// Background integrity verification — runs every 5 minutes
 	go runIntegrityCheck(context.Background(), mailStore, logger)
@@ -272,7 +288,8 @@ func main() {
 }
 
 // submitToWorker parses a raw email and submits it to the async index worker.
-func submitToWorker(worker *index.IndexWorker, store *storage.Store, raw []byte, id string, logger *slog.Logger) {
+// tenantID may be nil for global context.
+func submitToWorker(worker *index.TenantIndexWorker, store *storage.Store, raw []byte, id string, tenantID *int64, logger *slog.Logger) {
 	pm, err := mailparser.Parse(raw)
 	if err != nil {
 		logger.Warn("index: parse failed, skipping indexing", "id", id, "err", err)
@@ -296,6 +313,7 @@ func submitToWorker(worker *index.IndexWorker, store *storage.Store, raw []byte,
 		HasAttachment: len(pm.Attachments) > 0,
 		Date:          pm.Date,
 		Size:          int64(len(raw)),
+		TenantID:      tenantID,
 	}
 
 	worker.Submit(doc)
@@ -307,8 +325,9 @@ func submitToWorker(worker *index.IndexWorker, store *storage.Store, raw []byte,
 }
 
 // runBackfill walks the store, inserts missing DB metadata, and indexes
-// emails that have not yet been indexed.
-func runBackfill(ctx context.Context, store *storage.Store, idx index.Indexer, worker *index.IndexWorker, logger *slog.Logger) {
+// emails that have not yet been indexed. Per-tenant indexing is handled by
+// looking up each email's tenant_id from the DB.
+func runBackfill(ctx context.Context, store *storage.Store, idx index.Indexer, worker *index.TenantIndexWorker, logger *slog.Logger) {
 	logger.Info("backfill: starting")
 
 	count := 0
@@ -345,7 +364,8 @@ func runBackfill(ctx context.Context, store *storage.Store, idx index.Indexer, w
 
 		if !alreadyIndexed {
 			needIndex++
-			submitToWorker(worker, store, raw, id, logger)
+			tenantID, _ := store.GetTenantForMail(ctx, id)
+			submitToWorker(worker, store, raw, id, tenantID, logger)
 		}
 
 		if count%100 == 0 {
@@ -363,6 +383,68 @@ func runBackfill(ctx context.Context, store *storage.Store, idx index.Indexer, w
 	logger.Info("backfill: complete", "total", count, "submitted_for_index", needIndex, "errors", errCount)
 }
 
+// reindexTenant re-indexes all emails belonging to a specific tenant.
+// Used during migration when switching from global index to per-tenant indexes.
+func reindexTenant(ctx context.Context, store *storage.Store, mgr *index.TenantIndexManager, tenantID int64, logger *slog.Logger) error {
+	tid := tenantID
+	ids, err := store.GetAllIDsByTenant(ctx, &tid)
+	if err != nil {
+		return fmt.Errorf("reindex tenant %d: get IDs: %w", tenantID, err)
+	}
+
+	logger.Info("reindex tenant: starting", "tenant_id", tenantID, "count", len(ids))
+
+	idx := mgr.ForTenant(&tid)
+	indexed := 0
+	errCount := 0
+
+	for _, id := range ids {
+		raw, err := store.Load(id)
+		if err != nil {
+			logger.Warn("reindex tenant: load failed", "tenant_id", tenantID, "id", id, "err", err)
+			errCount++
+			continue
+		}
+
+		pm, parseErr := mailparser.Parse(raw)
+		if parseErr != nil {
+			logger.Warn("reindex tenant: parse failed", "tenant_id", tenantID, "id", id, "err", parseErr)
+			errCount++
+			continue
+		}
+
+		var attachNames []string
+		for _, a := range pm.Attachments {
+			if a.Filename != "" {
+				attachNames = append(attachNames, a.Filename)
+			}
+		}
+
+		doc := index.MailDocument{
+			ID:            id,
+			From:          pm.From,
+			To:            strings.Join(pm.To, ", "),
+			Subject:       pm.Subject,
+			Body:          pm.TextBody,
+			AttachNames:   strings.Join(attachNames, " "),
+			HasAttachment: len(pm.Attachments) > 0,
+			Date:          pm.Date,
+			Size:          int64(len(raw)),
+			TenantID:      &tid,
+		}
+
+		if err := idx.IndexSync(doc); err != nil {
+			logger.Warn("reindex tenant: index failed", "tenant_id", tenantID, "id", id, "err", err)
+			errCount++
+			continue
+		}
+		indexed++
+	}
+
+	logger.Info("reindex tenant: complete", "tenant_id", tenantID, "indexed", indexed, "errors", errCount)
+	return nil
+}
+
 // runIntegrityCheck verifies all stored emails every 5 minutes by re-computing
 // their SHA-256 and comparing it to the stored file ID.
 func runIntegrityCheck(ctx context.Context, store *storage.Store, logger *slog.Logger) {
diff --git a/internal/api/ldap_tenants.go b/internal/api/ldap_tenants.go
index 9872a56..2534acd 100644
--- a/internal/api/ldap_tenants.go
+++ b/internal/api/ldap_tenants.go
@@ -385,6 +385,314 @@ func (s *Server) handleRemoveTenantDomain(w http.ResponseWriter, r *http.Request
 	w.WriteHeader(http.StatusNoContent)
 }
 
+// ── PROJ-23: Per-Tenant LDAP handlers (Phase B) ─────────────────────────────
+
+// SetTenantLDAP wires the per-tenant LDAP config store into the API server and
+// registers the tenant LDAP routes.
+func (s *Server) SetTenantLDAP(store *ldapcfg.TenantStore) {
+	s.tenantLdapStore = store
+
+	// domain_admin routes — tenant_id comes from JWT session, NOT from URL
+	s.mux.HandleFunc("GET /api/tenant/ldap", s.authAdmin(s.handleGetTenantLDAP))
+	s.mux.HandleFunc("PUT /api/tenant/ldap", s.authAdmin(s.handleSaveTenantLDAP))
+	s.mux.HandleFunc("DELETE /api/tenant/ldap", s.authAdmin(s.handleDeleteTenantLDAP))
+	s.mux.HandleFunc("POST /api/tenant/ldap/test", s.authAdmin(s.handleTestTenantLDAP))
+
+	// superadmin routes — tenant_id from URL parameter
+	s.mux.HandleFunc("GET /api/admin/tenants/{id}/ldap", s.authMiddleware(s.requireRole(userstore.RoleSuperAdmin, s.handleAdminGetTenantLDAP)))
+	s.mux.HandleFunc("PUT /api/admin/tenants/{id}/ldap", s.authMiddleware(s.requireRole(userstore.RoleSuperAdmin, s.handleAdminSaveTenantLDAP)))
+	s.mux.HandleFunc("DELETE /api/admin/tenants/{id}/ldap", s.authMiddleware(s.requireRole(userstore.RoleSuperAdmin, s.handleAdminDeleteTenantLDAP)))
+	s.mux.HandleFunc("POST /api/admin/tenants/{id}/ldap/test", s.authMiddleware(s.requireRole(userstore.RoleSuperAdmin, s.handleAdminTestTenantLDAP)))
+}
+
+// ── domain_admin handlers (own tenant) ──────────────────────────────────────
+
+func (s *Server) handleGetTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	sess := sessionFromCtx(r.Context())
+	if sess.TenantID == nil {
+		writeError(w, http.StatusBadRequest, "no tenant context")
+		return
+	}
+	cfg, err := s.tenantLdapStore.Get(r.Context(), *sess.TenantID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to load tenant ldap config")
+		return
+	}
+	if cfg == nil {
+		writeError(w, http.StatusNotFound, "no tenant ldap config")
+		return
+	}
+	writeJSON(w, http.StatusOK, cfg)
+}
+
+func (s *Server) handleSaveTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	sess := sessionFromCtx(r.Context())
+	if sess.TenantID == nil {
+		writeError(w, http.StatusBadRequest, "no tenant context")
+		return
+	}
+
+	var cfg ldapcfg.TenantLDAPConfig
+	if err := json.NewDecoder(r.Body).Decode(&cfg); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+	cfg.TenantID = *sess.TenantID
+
+	if err := s.tenantLdapStore.Save(r.Context(), cfg, sess.Username); err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to save tenant ldap config")
+		return
+	}
+
+	s.audlog.Log(audit.Entry{
+		EventType: "tenant_ldap_config_saved",
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Success:   true,
+		Detail:    "Mandant-LDAP-Konfiguration gespeichert",
+	})
+
+	saved, err := s.tenantLdapStore.Get(r.Context(), *sess.TenantID)
+	if err != nil || saved == nil {
+		writeJSON(w, http.StatusOK, map[string]bool{"ok": true})
+		return
+	}
+	writeJSON(w, http.StatusOK, saved)
+}
+
+func (s *Server) handleDeleteTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	sess := sessionFromCtx(r.Context())
+	if sess.TenantID == nil {
+		writeError(w, http.StatusBadRequest, "no tenant context")
+		return
+	}
+	if err := s.tenantLdapStore.Delete(r.Context(), *sess.TenantID); err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to delete tenant ldap config")
+		return
+	}
+
+	s.audlog.Log(audit.Entry{
+		EventType: "tenant_ldap_config_deleted",
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Success:   true,
+		Detail:    "Mandant-LDAP-Konfiguration gelöscht",
+	})
+
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (s *Server) handleTestTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	sess := sessionFromCtx(r.Context())
+	if sess.TenantID == nil {
+		writeError(w, http.StatusBadRequest, "no tenant context")
+		return
+	}
+
+	var body struct {
+		UseSaved bool `json:"use_saved"`
+		ldapcfg.TenantLDAPConfig
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	testCfg := s.buildTenantTestConfig(r, body.UseSaved, *sess.TenantID, body.TenantLDAPConfig)
+	if testCfg == nil {
+		writeError(w, http.StatusNotFound, "no tenant ldap config saved")
+		return
+	}
+
+	result := ldapauth.TestConnection(*testCfg)
+
+	s.audlog.Log(audit.Entry{
+		EventType: "tenant_ldap_connection_test",
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Success:   result.OK,
+		Detail:    result.Message,
+	})
+
+	writeJSON(w, http.StatusOK, result)
+}
+
+// ── superadmin handlers (arbitrary tenant) ──────────────────────────────────
+
+func (s *Server) handleAdminGetTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	id, err := parseTenantID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid tenant id")
+		return
+	}
+	cfg, err := s.tenantLdapStore.Get(r.Context(), id)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to load tenant ldap config")
+		return
+	}
+	if cfg == nil {
+		writeError(w, http.StatusNotFound, "no tenant ldap config")
+		return
+	}
+	writeJSON(w, http.StatusOK, cfg)
+}
+
+func (s *Server) handleAdminSaveTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	id, err := parseTenantID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid tenant id")
+		return
+	}
+
+	var cfg ldapcfg.TenantLDAPConfig
+	if err := json.NewDecoder(r.Body).Decode(&cfg); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+	cfg.TenantID = id
+
+	sess := sessionFromCtx(r.Context())
+	if err := s.tenantLdapStore.Save(r.Context(), cfg, sess.Username); err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to save tenant ldap config")
+		return
+	}
+
+	s.audlog.Log(audit.Entry{
+		EventType: "tenant_ldap_config_saved",
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Success:   true,
+		Detail:    "Mandant-LDAP-Konfiguration gespeichert (tenant " + strconv.FormatInt(id, 10) + ")",
+	})
+
+	saved, err := s.tenantLdapStore.Get(r.Context(), id)
+	if err != nil || saved == nil {
+		writeJSON(w, http.StatusOK, map[string]bool{"ok": true})
+		return
+	}
+	writeJSON(w, http.StatusOK, saved)
+}
+
+func (s *Server) handleAdminDeleteTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	id, err := parseTenantID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid tenant id")
+		return
+	}
+	if err := s.tenantLdapStore.Delete(r.Context(), id); err != nil {
+		writeError(w, http.StatusInternalServerError, "failed to delete tenant ldap config")
+		return
+	}
+
+	sess := sessionFromCtx(r.Context())
+	s.audlog.Log(audit.Entry{
+		EventType: "tenant_ldap_config_deleted",
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Success:   true,
+		Detail:    "Mandant-LDAP-Konfiguration gelöscht (tenant " + strconv.FormatInt(id, 10) + ")",
+	})
+
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (s *Server) handleAdminTestTenantLDAP(w http.ResponseWriter, r *http.Request) {
+	if s.tenantLdapStore == nil {
+		writeError(w, http.StatusServiceUnavailable, "tenant ldap store not available")
+		return
+	}
+	id, err := parseTenantID(r)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid tenant id")
+		return
+	}
+
+	var body struct {
+		UseSaved bool `json:"use_saved"`
+		ldapcfg.TenantLDAPConfig
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	testCfg := s.buildTenantTestConfig(r, body.UseSaved, id, body.TenantLDAPConfig)
+	if testCfg == nil {
+		writeError(w, http.StatusNotFound, "no tenant ldap config saved")
+		return
+	}
+
+	result := ldapauth.TestConnection(*testCfg)
+
+	sess := sessionFromCtx(r.Context())
+	s.audlog.Log(audit.Entry{
+		EventType: "tenant_ldap_connection_test",
+		Username:  sess.Username,
+		IPAddress: remoteIP(r),
+		Success:   result.OK,
+		Detail:    result.Message + " (tenant " + strconv.FormatInt(id, 10) + ")",
+	})
+
+	writeJSON(w, http.StatusOK, result)
+}
+
+// buildTenantTestConfig constructs an ldapauth.Config for testing, either from
+// the saved tenant config or from the provided request body.
+func (s *Server) buildTenantTestConfig(r *http.Request, useSaved bool, tenantID int64, provided ldapcfg.TenantLDAPConfig) *ldapauth.Config {
+	if useSaved {
+		saved, err := s.tenantLdapStore.GetWithPassword(r.Context(), tenantID)
+		if err != nil || saved == nil {
+			return nil
+		}
+		return &ldapauth.Config{
+			URL:           saved.URL,
+			BindDN:        saved.BindDN,
+			BindPassword:  saved.BindPassword,
+			BaseDN:        saved.BaseDN,
+			UserFilter:    saved.UserFilter,
+			TLS:           saved.TLS,
+			TLSSkipVerify: saved.TLSSkipVerify,
+		}
+	}
+	return &ldapauth.Config{
+		URL:           provided.URL,
+		BindDN:        provided.BindDN,
+		BindPassword:  provided.BindPassword,
+		BaseDN:        provided.BaseDN,
+		UserFilter:    provided.UserFilter,
+		TLS:           provided.TLS,
+		TLSSkipVerify: provided.TLSSkipVerify,
+	}
+}
+
 // ── helpers ──────────────────────────────────────────────────────────────────
 
 func parseTenantID(r *http.Request) (int64, error) {
diff --git a/internal/api/server.go b/internal/api/server.go
index 20bbded..1d91d45 100644
--- a/internal/api/server.go
+++ b/internal/api/server.go
@@ -78,8 +78,10 @@ type Server struct {
 	pop3Store     *pop3store.Store
 	pop3Importer  *pop3store.Importer
 	uploadJobs    sync.Map // jobID → *UploadJob
-	ldapStore     *ldapcfg.Store
-	tenantStore   *tenantstore.Store
+	ldapStore         *ldapcfg.Store
+	tenantStore       *tenantstore.Store
+	tenantLdapStore   *ldapcfg.TenantStore   // PROJ-23: per-tenant LDAP config
+	idxMgr            *index.TenantIndexManager // PROJ-21 Phase 4: per-tenant Xapian index
 }
 
 // SetSMTPDaemon wires the SMTP daemon into the API server after construction.
@@ -100,6 +102,11 @@ func (s *Server) SetPop3(store *pop3store.Store, importer *pop3store.Importer) {
 	s.pop3Importer = importer
 }
 
+// SetIndexManager wires the per-tenant index manager into the API server (PROJ-21 Phase 4).
+func (s *Server) SetIndexManager(mgr *index.TenantIndexManager) {
+	s.idxMgr = mgr
+}
+
 // New creates and wires up a new API server.
 func New(
 	cfg config.APIConfig,
@@ -615,15 +622,26 @@ func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	result, err := s.idx.Search(req)
+	// PROJ-21 Phase 4: Use per-tenant index when available; fall back to
+	// global index + post-filter when the tenant index manager is not wired.
+	tenantID := tenantFromCtx(r.Context())
+	searchIdx := s.idx
+	usedTenantIndex := false
+	if s.idxMgr != nil && tenantID != nil {
+		searchIdx = s.idxMgr.ForTenant(tenantID)
+		usedTenantIndex = true
+	}
+
+	result, err := searchIdx.Search(req)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, "search failed")
 		return
 	}
 
-	// Tenant isolation: filter results to only this tenant's emails.
-	tenantID := tenantFromCtx(r.Context())
-	if tenantID != nil && len(result.Hits) > 0 {
+	// Fallback tenant isolation: post-filter when we used the global index
+	// but the user belongs to a tenant. This is the legacy path; the per-tenant
+	// index path above makes this unnecessary.
+	if tenantID != nil && !usedTenantIndex && len(result.Hits) > 0 {
 		allowedIDs, idErr := s.store.GetAllIDsByTenant(r.Context(), tenantID)
 		if idErr == nil {
 			allowed := make(map[string]struct{}, len(allowedIDs))
diff --git a/internal/auth/auth.go b/internal/auth/auth.go
index e4c90e8..66b8254 100644
--- a/internal/auth/auth.go
+++ b/internal/auth/auth.go
@@ -6,6 +6,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"strings"
 	"time"
 
 	"github.com/golang-jwt/jwt/v5"
@@ -24,11 +25,20 @@ type Session struct {
 	TenantSlug string
 }
 
+// TenantDomainLookup is the interface used by Manager to resolve an email domain
+// to a tenant ID. Implemented by tenantstore.Store.
+type TenantDomainLookup interface {
+	// GetTenantIDByDomain returns the tenant_id for a given domain, or nil if not found.
+	GetTenantIDByDomain(ctx context.Context, domain string) (*int64, error)
+}
+
 // Manager handles login, token issuance, validation, and logout.
 type Manager struct {
-	store     *userstore.Store
-	ldapStore *ldapcfg.Store
-	jwtSecret []byte
+	store           *userstore.Store
+	ldapStore       *ldapcfg.Store
+	jwtSecret       []byte
+	tenantLdapStore *ldapcfg.TenantStore   // PROJ-23: per-tenant LDAP config
+	tenantLookup    TenantDomainLookup     // PROJ-23: domain -> tenant_id resolution
 }
 
 // New creates a new auth Manager.
@@ -41,6 +51,13 @@ func New(store *userstore.Store, ldapStore *ldapcfg.Store, jwtSecret string) *Ma
 	}
 }
 
+// SetTenantLDAP wires the per-tenant LDAP config store and tenant domain lookup
+// into the auth manager. Both may be nil to disable per-tenant LDAP.
+func (m *Manager) SetTenantLDAP(tenantLdapStore *ldapcfg.TenantStore, tenantLookup TenantDomainLookup) {
+	m.tenantLdapStore = tenantLdapStore
+	m.tenantLookup = tenantLookup
+}
+
 // Login verifies credentials and returns a signed JWT token.
 // It first attempts a local password check. If that fails and LDAP is
 // configured and enabled, it falls back to LDAP authentication.
@@ -51,7 +68,7 @@ func (m *Manager) Login(username, password string) (string, *userstore.User, err
 		return m.issueToken(user)
 	}
 
-	// 2. LDAP fallback when the store is wired and the config is enabled.
+	// 2. Global LDAP fallback when the store is wired and the config is enabled.
 	if m.ldapStore != nil {
 		cfg, ldapErr := m.ldapStore.GetWithPassword(context.Background())
 		if ldapErr == nil && cfg != nil && cfg.Enabled {
@@ -93,6 +110,54 @@ func (m *Manager) Login(username, password string) (string, *userstore.User, err
 		}
 	}
 
+	// 3. PROJ-23: Per-tenant LDAP fallback — extract domain from username,
+	// look up the tenant, and try LDAP auth with that tenant's config.
+	if m.tenantLdapStore != nil && m.tenantLookup != nil {
+		if domain := extractDomain(username); domain != "" {
+			ctx := context.Background()
+			tenantID, lookupErr := m.tenantLookup.GetTenantIDByDomain(ctx, domain)
+			if lookupErr == nil && tenantID != nil {
+				tcfg, tErr := m.tenantLdapStore.GetWithPassword(ctx, *tenantID)
+				if tErr == nil && tcfg != nil && tcfg.Enabled {
+					attrs, authErr := ldapauth.Authenticate(ldapauth.Config{
+						URL:           tcfg.URL,
+						BindDN:        tcfg.BindDN,
+						BindPassword:  tcfg.BindPassword,
+						BaseDN:        tcfg.BaseDN,
+						UserFilter:    tcfg.UserFilter,
+						TLS:           tcfg.TLS,
+						TLSSkipVerify: tcfg.TLSSkipVerify,
+					}, username, password)
+					if authErr == nil {
+						role := tcfg.DefaultRole
+						if role == "" {
+							role = userstore.RoleUser
+						}
+						memberOf := attrs["memberOf"]
+						if memberOf != "" {
+							for _, gm := range tcfg.GroupMappings {
+								if gm.GroupDN != "" && containsGroup(memberOf, gm.GroupDN) {
+									role = gm.Role
+									break
+								}
+							}
+						}
+
+						email := attrs["mail"]
+						if email == "" {
+							email = username
+						}
+
+						ldapUser, upsertErr := m.store.UpsertLDAPUser(username, email, role, tenantID)
+						if upsertErr == nil {
+							return m.issueToken(ldapUser)
+						}
+					}
+				}
+			}
+		}
+	}
+
 	return "", nil, fmt.Errorf("auth: login: invalid credentials")
 }
 
@@ -281,6 +346,16 @@ func trimSpace(s string) string {
 	return s[start:end]
 }
 
+// extractDomain returns the domain part from a username containing '@'.
+// Returns empty string if no '@' is found.
+func extractDomain(username string) string {
+	idx := strings.LastIndex(username, "@")
+	if idx < 0 || idx == len(username)-1 {
+		return ""
+	}
+	return strings.ToLower(username[idx+1:])
+}
+
 // generateJTI returns a cryptographically random identifier for a JWT.
 func generateJTI() string {
 	b := make([]byte, 16)
diff --git a/internal/index/index.go b/internal/index/index.go
index 4cbfa9f..a19d8ce 100644
--- a/internal/index/index.go
+++ b/internal/index/index.go
@@ -16,6 +16,7 @@ type MailDocument struct {
 	HasAttachment bool
 	Date          time.Time
 	Size          int64
+	TenantID      *int64 // nil = global / superadmin context
 }
 
 // SearchRequest specifies search parameters.
diff --git a/internal/index/tenant_manager.go b/internal/index/tenant_manager.go
new file mode 100644
index 0000000..82c8fa7
--- /dev/null
+++ b/internal/index/tenant_manager.go
@@ -0,0 +1,110 @@
+package index
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+)
+
+// TenantIndexManager manages a pool of Xapian indexes, one per tenant.
+// Tenant 0 / nil maps to the global index used by superadmin and as a fallback.
+type TenantIndexManager struct {
+	basePath  string
+	batchSize int
+	backend   string
+	mu        sync.RWMutex
+	pool      map[int64]Indexer // tenant_id -> Indexer
+	global    Indexer           // tenant_id == 0 / nil (superadmin, fallback)
+}
+
+// NewTenantIndexManager creates a new TenantIndexManager.
+// The global index lives at basePath directly; per-tenant indexes at basePath/tenant-<id>/.
+func NewTenantIndexManager(basePath string, batchSize int, backend string) (*TenantIndexManager, error) {
+	// Ensure the base path directory exists.
+	if err := os.MkdirAll(basePath, 0o750); err != nil {
+		return nil, fmt.Errorf("tenant index manager: mkdir base: %w", err)
+	}
+
+	global, err := New(basePath, batchSize, backend)
+	if err != nil {
+		return nil, fmt.Errorf("tenant index manager: open global: %w", err)
+	}
+
+	return &TenantIndexManager{
+		basePath:  basePath,
+		batchSize: batchSize,
+		backend:   backend,
+		pool:      make(map[int64]Indexer),
+		global:    global,
+	}, nil
+}
+
+// ForTenant returns the Indexer for the given tenant.
+// nil or pointer to 0 returns the global index.
+// Other tenant IDs lazily open a per-tenant index at basePath/tenant-<id>/.
+func (m *TenantIndexManager) ForTenant(tenantID *int64) Indexer {
+	if tenantID == nil || *tenantID == 0 {
+		return m.global
+	}
+	id := *tenantID
+
+	// Fast path: read lock.
+	m.mu.RLock()
+	if idx, ok := m.pool[id]; ok {
+		m.mu.RUnlock()
+		return idx
+	}
+	m.mu.RUnlock()
+
+	// Slow path: write lock, create if not exists.
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Double check after acquiring write lock.
+	if idx, ok := m.pool[id]; ok {
+		return idx
+	}
+
+	dir := filepath.Join(m.basePath, fmt.Sprintf("tenant-%d", id))
+	if err := os.MkdirAll(dir, 0o750); err != nil {
+		// Return global as fallback on error.
+		return m.global
+	}
+
+	idx, err := New(dir, m.batchSize, m.backend)
+	if err != nil {
+		// Return global as fallback on error.
+		return m.global
+	}
+
+	m.pool[id] = idx
+	return idx
+}
+
+// Global returns the global (non-tenant) index.
+func (m *TenantIndexManager) Global() Indexer {
+	return m.global
+}
+
+// Close closes all indexes (global + per-tenant).
+func (m *TenantIndexManager) Close() error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	var firstErr error
+	for id, idx := range m.pool {
+		if err := idx.Close(); err != nil && firstErr == nil {
+			firstErr = fmt.Errorf("close tenant-%d index: %w", id, err)
+		}
+		delete(m.pool, id)
+	}
+
+	if m.global != nil {
+		if err := m.global.Close(); err != nil && firstErr == nil {
+			firstErr = fmt.Errorf("close global index: %w", err)
+		}
+	}
+
+	return firstErr
+}
diff --git a/internal/index/tenant_worker.go b/internal/index/tenant_worker.go
new file mode 100644
index 0000000..680f994
--- /dev/null
+++ b/internal/index/tenant_worker.go
@@ -0,0 +1,90 @@
+package index
+
+import (
+	"log/slog"
+	"sync"
+)
+
+// TenantIndexWorker processes MailDocument indexing requests asynchronously,
+// routing each document to the correct per-tenant Xapian index via TenantIndexManager.
+type TenantIndexWorker struct {
+	mgr    *TenantIndexManager
+	queue  chan MailDocument
+	done   chan struct{}
+	wg     sync.WaitGroup
+	logger *slog.Logger
+}
+
+// NewTenantWorker creates a new TenantIndexWorker with the given queue capacity.
+func NewTenantWorker(mgr *TenantIndexManager, queueSize int, logger *slog.Logger) *TenantIndexWorker {
+	if queueSize <= 0 {
+		queueSize = 1000
+	}
+	return &TenantIndexWorker{
+		mgr:    mgr,
+		queue:  make(chan MailDocument, queueSize),
+		done:   make(chan struct{}),
+		logger: logger,
+	}
+}
+
+// Submit enqueues a document for background indexing. If the queue is full,
+// the document is dropped and a warning is logged.
+func (w *TenantIndexWorker) Submit(doc MailDocument) {
+	select {
+	case w.queue <- doc:
+		// queued
+	default:
+		w.logger.Warn("tenant index worker: queue full, dropping document", "id", doc.ID)
+	}
+}
+
+// Start launches the background goroutine that processes the queue.
+func (w *TenantIndexWorker) Start() {
+	w.wg.Add(1)
+	go func() {
+		defer w.wg.Done()
+		w.logger.Info("tenant index worker: started", "queue_size", cap(w.queue))
+		for {
+			select {
+			case doc, ok := <-w.queue:
+				if !ok {
+					return
+				}
+				w.indexDoc(doc)
+			case <-w.done:
+				// Drain remaining items in the queue before exiting.
+				for {
+					select {
+					case doc, ok := <-w.queue:
+						if !ok {
+							return
+						}
+						w.indexDoc(doc)
+					default:
+						return
+					}
+				}
+			}
+		}
+	}()
+}
+
+// Stop signals the worker to drain remaining items and stop.
+func (w *TenantIndexWorker) Stop() {
+	close(w.done)
+	w.wg.Wait()
+	w.logger.Info("tenant index worker: stopped")
+}
+
+// QueueLen returns the current number of items waiting in the queue.
+func (w *TenantIndexWorker) QueueLen() int {
+	return len(w.queue)
+}
+
+func (w *TenantIndexWorker) indexDoc(doc MailDocument) {
+	idx := w.mgr.ForTenant(doc.TenantID)
+	if err := idx.IndexSync(doc); err != nil {
+		w.logger.Error("tenant index worker: index failed", "id", doc.ID, "tenant_id", doc.TenantID, "err", err)
+	}
+}
diff --git a/internal/ldapconfig/tenant_store.go b/internal/ldapconfig/tenant_store.go
new file mode 100644
index 0000000..c8e9f57
--- /dev/null
+++ b/internal/ldapconfig/tenant_store.go
@@ -0,0 +1,255 @@
+// Package ldapconfig — TenantStore manages per-tenant LDAP/AD configuration
+// stored in the tenant_ldap table. The bind password is encrypted with
+// AES-256-GCM using a SHA-256 derived key, identical to the global Store.
+package ldapconfig
+
+import (
+	"context"
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rand"
+	"crypto/sha256"
+	"encoding/json"
+	"fmt"
+	"io"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// TenantLDAPConfig is the persisted LDAP/AD configuration for a single tenant.
+type TenantLDAPConfig struct {
+	TenantID      int64          `json:"tenant_id"`
+	Enabled       bool           `json:"enabled"`
+	URL           string         `json:"url"`
+	BindDN        string         `json:"bind_dn"`
+	BindPassword  string         `json:"bind_password"`
+	BaseDN        string         `json:"base_dn"`
+	UserFilter    string         `json:"user_filter"`
+	TLS           bool           `json:"tls"`
+	TLSSkipVerify bool           `json:"tls_skip_verify"`
+	DefaultRole   string         `json:"default_role"`
+	GroupMappings []GroupMapping `json:"group_mappings"`
+}
+
+// TenantLDAPSummary is the abbreviated view returned by ListAll (for superadmin).
+type TenantLDAPSummary struct {
+	TenantID   int64  `json:"tenant_id"`
+	TenantName string `json:"tenant_name"`
+	Enabled    bool   `json:"enabled"`
+	URL        string `json:"url"`
+}
+
+// TenantStore manages per-tenant LDAP configuration persistence.
+type TenantStore struct {
+	pool   *pgxpool.Pool
+	encKey [32]byte
+}
+
+// NewTenantStore connects to PostgreSQL and returns a TenantStore.
+// The tenant_ldap table is created by the tenantstore package schema migration.
+// keyHex is the hex-encoded AES key (same as used for the global LDAP store).
+func NewTenantStore(dsn, keyHex string) (*TenantStore, error) {
+	ctx := context.Background()
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		return nil, fmt.Errorf("tenant ldap store: connect: %w", err)
+	}
+	key := sha256.Sum256([]byte(keyHex))
+	return &TenantStore{pool: pool, encKey: key}, nil
+}
+
+// Close releases the underlying connection pool.
+func (s *TenantStore) Close() {
+	s.pool.Close()
+}
+
+// Get returns the LDAP configuration for a tenant with the bind password masked.
+// Returns nil, nil when no configuration exists.
+func (s *TenantStore) Get(ctx context.Context, tenantID int64) (*TenantLDAPConfig, error) {
+	cfg, err := s.query(ctx, tenantID)
+	if err != nil {
+		return nil, err
+	}
+	if cfg == nil {
+		return nil, nil
+	}
+	if cfg.BindPassword != "" {
+		cfg.BindPassword = "\u2022\u2022\u2022\u2022\u2022\u2022"
+	}
+	return cfg, nil
+}
+
+// GetWithPassword returns the LDAP configuration including the decrypted bind password.
+// Returns nil, nil when no configuration exists.
+func (s *TenantStore) GetWithPassword(ctx context.Context, tenantID int64) (*TenantLDAPConfig, error) {
+	return s.query(ctx, tenantID)
+}
+
+// Save upserts the LDAP configuration for a tenant.
+// When bindPassword is empty the existing stored password is preserved.
+func (s *TenantStore) Save(ctx context.Context, cfg TenantLDAPConfig, updatedBy string) error {
+	mappingsJSON, err := json.Marshal(cfg.GroupMappings)
+	if err != nil {
+		return fmt.Errorf("tenant ldap store: marshal group_mappings: %w", err)
+	}
+
+	var encryptedPw []byte
+	if cfg.BindPassword != "" {
+		encryptedPw, err = s.encrypt(cfg.BindPassword)
+		if err != nil {
+			return fmt.Errorf("tenant ldap store: encrypt password: %w", err)
+		}
+	} else {
+		// Preserve existing password.
+		existing, qErr := s.query(ctx, cfg.TenantID)
+		if qErr == nil && existing != nil && existing.BindPassword != "" {
+			encryptedPw, err = s.encrypt(existing.BindPassword)
+			if err != nil {
+				return fmt.Errorf("tenant ldap store: re-encrypt existing password: %w", err)
+			}
+		}
+	}
+
+	_, err = s.pool.Exec(ctx, `
+		INSERT INTO tenant_ldap
+			(tenant_id, enabled, url, bind_dn, bind_password, base_dn, user_filter,
+			 tls, tls_skip_verify, default_role, group_mappings)
+		VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11)
+		ON CONFLICT (tenant_id) DO UPDATE SET
+			enabled         = EXCLUDED.enabled,
+			url             = EXCLUDED.url,
+			bind_dn         = EXCLUDED.bind_dn,
+			bind_password   = CASE WHEN EXCLUDED.bind_password IS NULL THEN tenant_ldap.bind_password ELSE EXCLUDED.bind_password END,
+			base_dn         = EXCLUDED.base_dn,
+			user_filter     = EXCLUDED.user_filter,
+			tls             = EXCLUDED.tls,
+			tls_skip_verify = EXCLUDED.tls_skip_verify,
+			default_role    = EXCLUDED.default_role,
+			group_mappings  = EXCLUDED.group_mappings
+	`,
+		cfg.TenantID, cfg.Enabled, cfg.URL, cfg.BindDN, encryptedPw, cfg.BaseDN,
+		cfg.UserFilter, cfg.TLS, cfg.TLSSkipVerify, cfg.DefaultRole,
+		string(mappingsJSON),
+	)
+	if err != nil {
+		return fmt.Errorf("tenant ldap store: upsert: %w", err)
+	}
+	return nil
+}
+
+// Delete removes the LDAP configuration for a tenant.
+func (s *TenantStore) Delete(ctx context.Context, tenantID int64) error {
+	_, err := s.pool.Exec(ctx, `DELETE FROM tenant_ldap WHERE tenant_id = $1`, tenantID)
+	return err
+}
+
+// ListAll returns a summary of all tenant LDAP configurations (for superadmin).
+func (s *TenantStore) ListAll(ctx context.Context) ([]TenantLDAPSummary, error) {
+	rows, err := s.pool.Query(ctx, `
+		SELECT tl.tenant_id, t.name, tl.enabled, tl.url
+		FROM tenant_ldap tl
+		JOIN tenants t ON t.id = tl.tenant_id
+		ORDER BY tl.tenant_id
+	`)
+	if err != nil {
+		return nil, fmt.Errorf("tenant ldap store: list all: %w", err)
+	}
+	defer rows.Close()
+
+	var summaries []TenantLDAPSummary
+	for rows.Next() {
+		var s TenantLDAPSummary
+		if err := rows.Scan(&s.TenantID, &s.TenantName, &s.Enabled, &s.URL); err != nil {
+			return nil, fmt.Errorf("tenant ldap store: scan: %w", err)
+		}
+		summaries = append(summaries, s)
+	}
+	if summaries == nil {
+		summaries = []TenantLDAPSummary{}
+	}
+	return summaries, rows.Err()
+}
+
+// query reads the tenant_ldap row for a tenant and decrypts the bind password.
+func (s *TenantStore) query(ctx context.Context, tenantID int64) (*TenantLDAPConfig, error) {
+	row := s.pool.QueryRow(ctx, `
+		SELECT tenant_id, enabled, url, bind_dn, bind_password, base_dn, user_filter,
+		       tls, tls_skip_verify, default_role, group_mappings
+		FROM tenant_ldap WHERE tenant_id = $1
+	`, tenantID)
+
+	var cfg TenantLDAPConfig
+	var encPw []byte
+	var mappingsRaw []byte
+
+	err := row.Scan(
+		&cfg.TenantID, &cfg.Enabled, &cfg.URL, &cfg.BindDN, &encPw,
+		&cfg.BaseDN, &cfg.UserFilter, &cfg.TLS, &cfg.TLSSkipVerify,
+		&cfg.DefaultRole, &mappingsRaw,
+	)
+	if err == pgx.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("tenant ldap store: query: %w", err)
+	}
+
+	if len(encPw) > 0 {
+		plain, err := s.decrypt(encPw)
+		if err != nil {
+			return nil, fmt.Errorf("tenant ldap store: decrypt password: %w", err)
+		}
+		cfg.BindPassword = plain
+	}
+
+	if len(mappingsRaw) > 0 {
+		if err := json.Unmarshal(mappingsRaw, &cfg.GroupMappings); err != nil {
+			return nil, fmt.Errorf("tenant ldap store: unmarshal group_mappings: %w", err)
+		}
+	}
+	if cfg.GroupMappings == nil {
+		cfg.GroupMappings = []GroupMapping{}
+	}
+
+	return &cfg, nil
+}
+
+// encrypt encrypts plaintext with AES-256-GCM using the store's key.
+func (s *TenantStore) encrypt(plaintext string) ([]byte, error) {
+	block, err := aes.NewCipher(s.encKey[:])
+	if err != nil {
+		return nil, err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return nil, err
+	}
+	nonce := make([]byte, gcm.NonceSize())
+	if _, err := io.ReadFull(rand.Reader, nonce); err != nil {
+		return nil, err
+	}
+	return gcm.Seal(nonce, nonce, []byte(plaintext), nil), nil
+}
+
+// decrypt decrypts ciphertext produced by encrypt.
+func (s *TenantStore) decrypt(ciphertext []byte) (string, error) {
+	block, err := aes.NewCipher(s.encKey[:])
+	if err != nil {
+		return "", err
+	}
+	gcm, err := cipher.NewGCM(block)
+	if err != nil {
+		return "", err
+	}
+	if len(ciphertext) < gcm.NonceSize() {
+		return "", fmt.Errorf("tenant ldap store: ciphertext too short")
+	}
+	nonce, data := ciphertext[:gcm.NonceSize()], ciphertext[gcm.NonceSize():]
+	plain, err := gcm.Open(nil, nonce, data, nil)
+	if err != nil {
+		return "", err
+	}
+	return string(plain), nil
+}
diff --git a/internal/tenantstore/store.go b/internal/tenantstore/store.go
index 0420caa..7a3249b 100644
--- a/internal/tenantstore/store.go
+++ b/internal/tenantstore/store.go
@@ -252,6 +252,20 @@ func (s *Store) GetByDomain(ctx context.Context, domain string) (*Tenant, error)
 	return &t, nil
 }
 
+// GetTenantIDByDomain returns the tenant_id for a given email domain.
+// Returns nil if no tenant is found. Satisfies the auth.TenantDomainLookup interface.
+func (s *Store) GetTenantIDByDomain(ctx context.Context, domain string) (*int64, error) {
+	t, err := s.GetByDomain(ctx, domain)
+	if err != nil {
+		return nil, err
+	}
+	if t == nil {
+		return nil, nil
+	}
+	id := t.ID
+	return &id, nil
+}
+
 // getDomain is a private helper to load a TenantDomain by its primary key.
 func (s *Store) getDomain(ctx context.Context, id int64) (*TenantDomain, error) {
 	row := s.pool.QueryRow(ctx,