Sendmail får ikke lov til at sende via TDC

Jeg har et problem med opsætningen af min Sendmail server. Jeg har sat serveren op til at bruge smtp.mail.dk som udgående mailserver. Når jeg prøver at sende en mail (via PHP), får jeg følgende meddelelse:

The original message was received at Wed, 27 Apr 2005 21:42:05 +0200
from localhost.localdomain [127.0.0.1]

  ----- The following addresses had permanent fatal errors -----
<fiskah@gmail.com>
    (reason: 550 <apache@localhost.localdomain>: Sender address rejected: Domain
not found)

  ----- Transcript of session follows -----
... while talking to smtp.mail.dk.:
>>> DATA
<<< 550 <apache@localhost.localdomain>: Sender address rejected: Domain not found
550 5.1.1 <philip@fcknet.dk>... User unknown
<<< 554 Error: no valid recipients

nedenunder står følgende uddybning:

Failed delivery status
Final recipient     fiskah@gmail.com
Reason for failure     550 <apache@localhost.localdomain>: Sender address rejected: Domain not found
Remote mail server     smtp.mail.dk
Reporting mail server     localhost.localdomain