sendmail - domain must exist

Jeg får denen fejl mail tilbage fra surftowns mail server:

Message text    

The original message was received at Sun, 4 Feb 2007 13:29:15 +0100
from mysqlhosting.dk [127.0.0.1]

  ----- The following addresses had permanent fatal errors -----
<min@email.net>
    (reason: 552 sorry, your envelope sender domain must exist (#5.7.1))

  ----- Transcript of session follows -----
... while talking to mail2.surftown.dk.:
>>> DATA
<<< 552 sorry, your envelope sender domain must exist (#5.7.1)
554 5.0.0 Service unavailable

Failed delivery status
Final recipient     min@email.net
Reason for failure     552 sorry, your envelope sender domain must exist (#5.7.1)
Remote mail server     mail2.surftown.dk
Reporting mail server     mysqlhosting.dk

Hvordan løser jeg mit problem? domænet ligger hos gratisdns på deres navneserver