Fixed it, see answer below
On my drupal installation there are some fancy webforms which send plain text emails, turning on the mime module messes them all up. But, I need the mime module to send html mails when using simplenews newsletters.
I did the following in the settings but it has no effect (html = plain): (http://abload.de/img/bildschirmfoto2013-11w1j7v.png)
What did I miss? How to use the mime mails only when simplenews is sending mails? I set HTML as format in newsletter settings.