OS X (or other OS's): how to get a fully-qualified domain name?

[John Owens]

Greetings, I'm using emacs 21.3.50 (built from CVS)
on OS X 10.3. I would like it to use a fully-qualified 
domain name when sending mail. Instead, it uses
system-name, which appears to just use my 
"computer name" (which is no way fully-qualified).

What would be ideal is if I could use the system-name
function to dynamically return where I am (since 
if I run emacs at home, then go to work, plug it in, and
use emacs there, I'd like emacs to use the work 
machine's domain name, not the one from home). I
figure that OS X knows its domain name (I can see it 
in the logs if I ssh somewhere, for instance), but I don't
know how to get that information nor do I know 
how to make emacs use it.

Suggestions?

JDO