relaying mail via comcast smtp server

[Robert La Ferla]

I need to set up outgoing mail on one of my Linux systems.  I configured 
masquerading via sendmail but I ran into this anti-SPAM measure from 
Comcast:

Diagnostic-Code: SMTP; 550 [PERMFAIL] comcast.net requires valid sender 
domain

I was able to trick it by adding my domain name to 127.0.0.1 
mydomain.com localhost but that's not a good solution because it 
interferes with other programs.

What's the best way to handle this?