> See 
> http://www.reitter-it-media.de/software/osxpostfix.html.
> I was at a conference recently with wireless
> access, so I could receive email, but couldn't
> send. This has been an ongoing problem... During
> the boring talks I located this article and was
> able to set up my laptop as an smtp server.  Even
> with my 10 years forgotten unix skills.  However,
> if you know no unix, beware.
> 
> Rob

Or if you want to do it the Mac way (no Unix skills required).
http://www.roadstead.com/weblog/Tutorials/PostfixEnabler.html
Rob