Current versions of OS X use Postfix as the mail server but you have to enable it.

For a detailed how-to, see:
http://www.stepwise.com/Articles/Workbench/eart.index.html

Or save yourself lots of frustration and time and spend $10 for Postfix Enabler and get running in a couple of minutes.
http://cutedgesystems.com/software/PostfixEnabler/