SMTP Server - Default Senders Address Invalid

I have a 2003 server suning IIS with Virtual SMTP server.

When messages are sent out bvia a smarthost they get bounced back by the
recipients mail system. The error says the senders address is
"administrator@192.168.0.200" - the ip address is the address of the 2003
server.

the from address of the email message is shown as postmaster.RemoveThis@mydomain.com -
where mydomain is the fully qualified domain name set out in the delivery tab
under IIS.

I need to understand why the senders address exists and where is it specified?