does anyone knows of any good software for Mac OS X or at least for RHEL/CENTOS Linux that I can run against my SMTP Server?
You can also try Xstress http://xstress.sourceforge.net/ although you will have to compile it from the source code.
From their Readme:
xstress is an SMTP Stress Testing Tool. Essentially xstress creates multiple connections to (your) mail server and sends multiple mails repeatedly over all connections.