Get error code 0xC00402C7 from SMTP

I'm running the SMTP service on a Windows 2003 Server for relaying the
internal mails to our mailhost (Internet-Provider). Unfortunately all non
delivery messages from our 2003 Server can't be delivered.
The reason is, that our Server is in a internal domain and sends the non
delivery messages with a sender address from this internal domain. Means,
that our mailhost does not accept any mail from a non existing
Internet-Domain (sender-address).

How can I change the sender address of our Windows 2003 Server, so that he
uses a sender-address like (E-Mail Removed) and not
(E-Mail Removed)l?

Thanks for every help!