Moin!
We have the same Problem... we have the same answer since some days... Exemple of the Error
<silvio.raffath <at>bluwin.ch>: host mxbw.bluewin.ch[195.186.19.144] said: 550 RCPT TO:<silvio.raffath <at> bluwin.ch> Relaying not allowed
This is most definately not a technical problem but rather a human problem (aka 'typo'). Have a close look at the recipient's domain and then you'll see that there's missing an 'e'.
cheers, - Fabian