SMTP from outside

[Paul Franklin]

We have a few employees who must send e-mail from off site on occasion.  
In one case, I can probably just add a privileged network, but in others 
the computer is portable and may be at a hotel whose IP address I don't 
know.  I would like to know how to set up for those computers to send 
messages from off site.  Is it possible to require authentication, and 
if so, how?
Thank you so much.
--Paul