diff --git a/go.mod b/go.mod
index 4700a303..49935675 100644
--- a/go.mod
+++ b/go.mod
@@ -5,4 +5,5 @@ go 1.18
 require (
 	github.com/emersion/go-message v0.18.1
 	github.com/emersion/go-sasl v0.0.0-20231106173351-e73c9f7bad43
+	golang.org/x/text v0.14.0
 )
diff --git a/go.sum b/go.sum
index 1a059484..ce241ed1 100644
--- a/go.sum
+++ b/go.sum
@@ -27,6 +27,7 @@ golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/text v0.3.3/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
 golang.org/x/text v0.3.7/go.mod h1:u+2+/6zg+i71rQMx5EYifcz6MCKuco9NR6JIITiCfzQ=
 golang.org/x/text v0.7.0/go.mod h1:mrYo+phRRbMaCq/xk9113O4dZlRixOauAjOtrjsXDZ8=
+golang.org/x/text v0.14.0 h1:ScX5w1eTa3QqT8oi6+ziP7dTV1S2+ALU0bI+0zXKWiQ=
 golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20191119224855-298f0cb1881e/go.mod h1:b+2E5dAYhXwXZwtnZ6UAqBI28+e2cm9otk0dWdXHAEo=
diff --git a/imapserver/capability.go b/imapserver/capability.go
index 37da104b..d76ebb8d 100644
--- a/imapserver/capability.go
+++ b/imapserver/capability.go
@@ -112,3 +112,12 @@ func addAvailableCaps(caps *[]imap.Cap, available imap.CapSet, l []imap.Cap) {
 		}
 	}
 }
+
+func (c *Conn) availableCapsSet() imap.CapSet {
+	caps := c.availableCaps()
+	capSet := make(imap.CapSet)
+	for _, cap := range caps {
+		capSet[cap] = struct{}{}
+	}
+	return capSet
+}
diff --git a/imapserver/search.go b/imapserver/search.go
index 91466818..7545f075 100644
--- a/imapserver/search.go
+++ b/imapserver/search.go
@@ -8,6 +8,8 @@ import (
 	"github.com/emersion/go-imap/v2"
 	"github.com/emersion/go-imap/v2/internal"
 	"github.com/emersion/go-imap/v2/internal/imapwire"
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
 )
 
 func (c *Conn) handleSearch(tag string, dec *imapwire.Decoder, numKind NumKind) error {
@@ -85,7 +87,22 @@ func (c *Conn) handleSearch(tag string, dec *imapwire.Decoder, numKind NumKind)
 		return err
 	}
 
-	if c.enabled.Has(imap.CapIMAP4rev2) || extended {
+	var supportsESEARCH bool
+	if capSession, ok := c.session.(SessionCapabilities); ok {
+		sessionCaps := capSession.GetCapabilities()
+		supportsESEARCH = sessionCaps.Has(imap.CapESearch) || sessionCaps.Has(imap.CapIMAP4rev2)
+	} else {
+		availableCaps := c.availableCaps()
+		for _, cap := range availableCaps {
+			if cap == imap.CapESearch || cap == imap.CapIMAP4rev2 {
+				supportsESEARCH = true
+				break
+			}
+		}
+	}
+
+	// Use ESEARCH format only if session supports it AND client used extended syntax
+	if supportsESEARCH && extended {
 		return c.writeESearch(tag, data, &options, numKind)
 	} else {
 		return c.writeSearch(data.All)
@@ -98,15 +115,17 @@ func (c *Conn) writeESearch(tag string, data *imap.SearchData, options *imap.Sea
 
 	enc.Atom("*").SP().Atom("ESEARCH")
 	if tag != "" {
-		enc.SP().Special('(').Atom("TAG").SP().Atom(tag).Special(')')
+		enc.SP().Special('(').Atom("TAG").SP().String(tag).Special(')')
 	}
 	if numKind == NumKindUID {
 		enc.SP().Atom("UID")
 	}
-	// When there is no result, we need to send an ESEARCH response with no ALL
-	// keyword
-	if options.ReturnAll && !isNumSetEmpty(data.All) {
-		enc.SP().Atom("ALL").SP().NumSet(data.All)
+	// When there is no result, we need to send an ESEARCH response with no sequence set after ALL.
+	if options.ReturnAll {
+		enc.SP().Atom("ALL")
+		if data.All != nil && !isNumSetEmpty(data.All) {
+			enc.SP().NumSet(data.All)
+		}
 	}
 	if options.ReturnMin && data.Min > 0 {
 		enc.SP().Atom("MIN").SP().Number(data.Min)
@@ -136,24 +155,28 @@ func (c *Conn) writeSearch(numSet imap.NumSet) error {
 	defer enc.end()
 
 	enc.Atom("*").SP().Atom("SEARCH")
-	var ok bool
-	switch numSet := numSet.(type) {
-	case imap.SeqSet:
-		var nums []uint32
-		nums, ok = numSet.Nums()
-		for _, num := range nums {
-			enc.SP().Number(num)
+
+	if numSet != nil {
+		var ok bool
+		switch numSet := numSet.(type) {
+		case imap.SeqSet:
+			var nums []uint32
+			nums, ok = numSet.Nums()
+			for _, num := range nums {
+				enc.SP().Number(num)
+			}
+		case imap.UIDSet:
+			var uids []imap.UID
+			uids, ok = numSet.Nums()
+			for _, uid := range uids {
+				enc.SP().UID(uid)
+			}
 		}
-	case imap.UIDSet:
-		var uids []imap.UID
-		uids, ok = numSet.Nums()
-		for _, uid := range uids {
-			enc.SP().UID(uid)
+		if !ok {
+			return fmt.Errorf("imapserver: failed to enumerate message numbers in SEARCH response (dynamic set?)")
 		}
 	}
-	if !ok {
-		return fmt.Errorf("imapserver: failed to enumerate message numbers in SEARCH response")
-	}
+
 	return enc.CRLF()
 }
 
@@ -178,7 +201,7 @@ func readSearchReturnOpts(dec *imapwire.Decoder, options *imap.SearchOptions) er
 		case "SAVE":
 			options.ReturnSave = true
 		default:
-			return newClientBugError("unknown SEARCH RETURN option")
+			// RFC 4731: A server MUST ignore any unrecognized return options.
 		}
 		return nil
 	})
@@ -196,7 +219,15 @@ func readSearchKey(criteria *imap.SearchCriteria, dec *imapwire.Decoder) error {
 		return readSearchKeyWithAtom(criteria, dec, key)
 	}
 	return dec.ExpectList(func() error {
-		return readSearchKey(criteria, dec)
+		for {
+			if err := readSearchKey(criteria, dec); err != nil {
+				return err
+			}
+			if !dec.SP() {
+				break
+			}
+		}
+		return nil
 	})
 }
 
@@ -241,7 +272,7 @@ func readSearchKeyWithAtom(criteria *imap.SearchCriteria, dec *imapwire.Decoder,
 			return dec.Err()
 		}
 		criteria.Header = append(criteria.Header, imap.SearchCriteriaHeaderField{
-			Key:   strings.Title(strings.ToLower(key)),
+			Key:   cases.Title(language.English).String(strings.ToLower(key)),
 			Value: value,
 		})
 	case "HEADER":
@@ -308,7 +339,7 @@ func readSearchKeyWithAtom(criteria *imap.SearchCriteria, dec *imapwire.Decoder,
 		}
 		var not imap.SearchCriteria
 		if err := readSearchKey(&not, dec); err != nil {
-			return nil
+			return err
 		}
 		criteria.Not = append(criteria.Not, not)
 	case "OR":
@@ -317,13 +348,13 @@ func readSearchKeyWithAtom(criteria *imap.SearchCriteria, dec *imapwire.Decoder,
 		}
 		var or [2]imap.SearchCriteria
 		if err := readSearchKey(&or[0], dec); err != nil {
-			return nil
+			return err
 		}
 		if !dec.ExpectSP() {
 			return dec.Err()
 		}
 		if err := readSearchKey(&or[1], dec); err != nil {
-			return nil
+			return err
 		}
 		criteria.Or = append(criteria.Or, or)
 	case "$":
@@ -339,5 +370,5 @@ func readSearchKeyWithAtom(criteria *imap.SearchCriteria, dec *imapwire.Decoder,
 }
 
 func searchKeyFlag(key string) imap.Flag {
-	return imap.Flag("\\" + strings.Title(strings.ToLower(key)))
+	return imap.Flag("\\" + cases.Title(language.English).String(strings.ToLower(key)))
 }
diff --git a/imapserver/session.go b/imapserver/session.go
index 35b40e8d..45a94e3d 100644
--- a/imapserver/session.go
+++ b/imapserver/session.go
@@ -124,3 +124,14 @@ type SessionAppendLimit interface {
 	// this server in an APPEND command.
 	AppendLimit() uint32
 }
+
+// SessionCapabilities is an IMAP session which can provide its current
+// capabilities for capability filtering.
+type SessionCapabilities interface {
+	Session
+
+	// GetCapabilities returns the session-specific capabilities.
+	// This allows sessions to filter capabilities based on client behavior
+	// or other session-specific factors.
+	GetCapabilities() imap.CapSet
+}