emails wont send out-

["Tom Warfield" <twarfield@astcorp.com>]

We have clients using Outlook (yes i know what losers) and were running
sendmail on debian.  Okay so this is the problem, when they send a email in
Outlook to a email address (for example 1231234567@mobile.att.net) Outlook
wont let them send it out.  Its not the message or anything like that
because if i change the address to something else the message goes, so its
specificaly the address.  Outlook connects to the server then it just sits
there.  Im thinking Sendmail doesnt like it for some reason and i cant
figure out why...anyone have any ideas.

Thanks,
Tom