Wrong reply-to header in emails (tine20)
I found the solution:

If you use two different servers for mail and tine20, you need to use the mail address if you create the Exchange Connection on the android device. The automatic set up will fail, and in the advanced settings you can enter the name of the tine20 server.

If you use the tine20 login on the first screen, the stock mail app will add a reply-to header to the tine20 server. And that's not valid, if you run two servers.