Ok. im completely confused now.
I installed Thunderbird and put in my Account. It worked out of the box. But the wierdness doesnt stop here. i then started The Bat and Outlook and the Mails stuck in there could be send aswell.
Whenever i send an Email with the Webinterface i still get the error 500 message BUT those Emails are send aswell. Which is even stranger…
And i definitely didnt touch my server since the last time i tried to send a Mail. No upgrades, no configchanges, no restarts. Nothing.
Gremlins?