After upgrading to kMail 5.7.0, sending mails does not work any more. Any sent mails queue in the outbox, triggering a notification "Fehler beim Übertragen der Nachricht. Serverfehler" without any other detail. Checking my mail server's logs, there is not even an connection attempt, so something must already go wrong in kMail or Akonadi. Obviously, this is a big issue for me...
Happening for me as well. It says either "Server error" or something about "mail transport failed". Nothing in any logs that I can find. The SMTP server used is GMail.
Have it as well. It was working before
*** This bug has been marked as a duplicate of bug 388068 ***