diff --git a/internal/rpc/channels_updates_rpc_test.go b/internal/rpc/channels_updates_rpc_test.go
index b05d0b08..57a60fe5 100644
--- a/internal/rpc/channels_updates_rpc_test.go
+++ b/internal/rpc/channels_updates_rpc_test.go
@@ -640,6 +640,39 @@ func TestChannelUnreadMentionsRPCUsesMentionState(t *testing.T) {
 	}
 }
 
+// A channel post containing an @token that is not a syntactically valid
+// username (too short, leading digit, etc.) must still send. Real Telegram
+// renders it as a mention and only fails when the reader taps it; it never
+// rejects the send. Regression for a 500 INTERNAL_SERVER_ERROR.
+func TestChannelSendMessageWithUnresolvableMentionSucceeds(t *testing.T) {
+	ctx := context.Background()
+	userStore := memory.NewUserStore()
+	owner, _ := userStore.Create(ctx, domain.User{AccessHash: 9201, Phone: "15550009201", FirstName: "Owner", Username: "owner_badmention"})
+	channelStore := memory.NewChannelStore()
+	channelService := appchannels.NewService(channelStore)
+	r := New(Config{}, Deps{
+		Users:    appusers.NewService(userStore),
+		Channels: channelService,
+	}, zaptest.NewLogger(t), clock.System)
+	created, err := channelService.CreateMegagroupFromCreateChat(ctx, owner.ID, domain.CreateChannelRequest{
+		Title: "Bad Mention",
+		Date:  1700009201,
+	})
+	if err != nil {
+		t.Fatalf("create megagroup: %v", err)
+	}
+	peer := &tg.InputPeerChannel{ChannelID: created.Channel.ID, AccessHash: created.Channel.AccessHash}
+	for i, text := range []string{"look at @zio", "hi @2cool and @_x"} {
+		if _, err := r.onMessagesSendMessage(WithUserID(ctx, owner.ID), &tg.MessagesSendMessageRequest{
+			Peer:     peer,
+			Message:  text,
+			RandomID: int64(9202001 + i),
+		}); err != nil {
+			t.Fatalf("send %q: unexpected error %v", text, err)
+		}
+	}
+}
+
 func TestChannelDifferenceIncludesExtraForwardSourceChannel(t *testing.T) {
 	channel := domain.Channel{ID: 2000000100, AccessHash: 9010, Title: "Megagroup", Megagroup: true, Date: 1700000000, Pts: 3}
 	source := domain.Channel{ID: 2000000101, AccessHash: 9011, Title: "Source", Broadcast: true, Date: 1700000000}
diff --git a/internal/rpc/messages_bot_no_state.go b/internal/rpc/messages_bot_no_state.go
index 8cedef6e..ad82ff36 100644
--- a/internal/rpc/messages_bot_no_state.go
+++ b/internal/rpc/messages_bot_no_state.go
@@ -341,7 +341,10 @@ func (r *Router) mentionedUserIDsFromDomainMessage(ctx context.Context, currentU
 		for _, username := range extractMentionUsernames(message, domain.MaxChannelMentionRecipients-len(out)) {
 			user, found, err := identity.ResolveUsername(ctx, currentUserID, username)
 			if err != nil {
-				return nil, internalErr()
+				if mentionResolveFatal(err) {
+					return nil, internalErr()
+				}
+				continue
 			}
 			if found {
 				add(user.ID)
diff --git a/internal/rpc/messages_send.go b/internal/rpc/messages_send.go
index 51cbc892..ec8d6880 100644
--- a/internal/rpc/messages_send.go
+++ b/internal/rpc/messages_send.go
@@ -437,7 +437,10 @@ func (r *Router) mentionedUserIDsFromMessage(ctx context.Context, currentUserID
 		for _, username := range extractMentionUsernames(message, domain.MaxChannelMentionRecipients-len(out)) {
 			user, found, err := identity.ResolveUsername(ctx, currentUserID, username)
 			if err != nil {
-				return nil, internalErr()
+				if mentionResolveFatal(err) {
+					return nil, internalErr()
+				}
+				continue
 			}
 			if found {
 				add(user.ID)
@@ -450,6 +453,17 @@ func (r *Router) mentionedUserIDsFromMessage(ctx context.Context, currentUserID
 	return out, nil
 }
 
+// mentionResolveFatal reports whether a ResolveUsername error while scanning
+// message text for @mentions should abort the send. A syntactically invalid or
+// unoccupied @token is not a failure: real Telegram sends the message, renders
+// the token as a mention, and only fails to open it when the reader taps it.
+// Only an unexpected (storage) error aborts the RPC.
+func mentionResolveFatal(err error) bool {
+	return err != nil &&
+		!errors.Is(err, domain.ErrUsernameInvalid) &&
+		!errors.Is(err, domain.ErrUsernameNotOccupied)
+}
+
 func extractMentionUsernames(message string, limit int) []string {
 	if limit <= 0 || message == "" {
 		return nil