Clean up destination queues when moving messages from Outbox

internal/imapserver/mailbox.go

@@ -326,6 +326,9 @@ func (mbox *Mailbox) MoveMessages(uid bool, seqset *imap.SeqSet, dest string) er
 		if err := mbox.backend.Storage.MailMove(mbox.name, int(id), dest); err != nil {
 			return err
 		}
+		if mbox.name == "Outbox" {
+			mbox.backend.Storage.QueueDeleteDestinationForID("Outbox", int(id))
+		}
 	}
 	return nil
 }