If you want to run a mail server for testing on mac osx you could do it the hard way and run Postfix on your local machine. I tought „Hey look in the Apache booth !“ and TaTa Apache James is to the rescue.
You have to run with sudo because osx don’t want the user to run stuff on lower ports. You’ll get some exceptions concerning the binding of the ports if you try.
All the basic configuration stuff is done in apps/james/SAR-INF/config.xml
Read more about James at the wiki.
On the client side beside a telnet session with the shell I use Thunderbird for mac osx because I don’t want to have my testing stuff in my productive mail.app