I wondered if you now know what is wrong?
My server gives this bounced mail only when I try to send it to another server.The mail I send to myself still arives!
For some reason this mail works fine, but doesn't arrive?
Email adres sender doesn't exists?
... while talking to mail.somehost.com.:
>>> MAIL From:<email@example.com> SIZE=46
<<< 501 <nobody@localhost>... Sender domain must exist
501 firstname.lastname@example.org... Data format error