Sorry to spam the list for this, but I was not able to find anything close to a technical support email contact on their web page...

To avoid many round trip mails, here's the problem:

we get incoming mails with senders like +4176...@mms.sunrise.ch.

But:

mms.sunrise.ch. 3600 IN MX 10 mm3.sunrise.ch.

Host mm3.sunrise.ch not found: 3(NXDOMAIN)

So, those sender addresses are invalid, and get rejected on our mail servers. Might want to fix your DNS..

Cheers, Markus