Well, THIS was the best example for a layer 8 problem!
As I suggested in the previous post, the hostname in the DNS Query looked strange.
Here's the problem/solution.
The result for nslookup -querytype=mx was:
hostname (which is correct) just attached my domain at the end (which is wrong, as the dns query doesn't give out the same as the reverse dns query)
Okay, now I fixed it, and no mail is bounced or anything else. Just a problem with corrupt headers, but this is also a layer 8 problem, as only mails sent by a do-it-yourself-contact-form have corrupt header.
Thanks for your help.