Mail routing

["John R. Sheets" <dusk@ravendusk.org>]

Hiya,

I'm trying to get email working properly from work here, but I'm having
trouble avoiding anti-relaying filters.  The quick rundown of my setup is
that I'm using a virtual domain, ravendusk.org, hosted by PC Online; my 
mailer is mutt, combined with fetchmail and procmail.  At home, when I
dial directly into PC Online, things are fine.  But when I try to send
SMTP from work, from our codeweavers.com domain, PC Online's mail server
rejects it because it looks like I'm trying to relay.  

I've tried any number of header re-writing (Mutt is nice that way), but
with no success.  Something is still not matching up correctly.

Does anyone have any advice, or just plain background on what may be
going awry?

Thanks,
John