If by monitoring you refer to parsing message content for compliance with organizations standards, etc.: I don't think Nagios is especially well-suited for that, although you could probably create a plug-in that would do so. I really wouldn't recommend it, however. You would be better off finding a different tool better suited for this purpose.
With regard to e-mail server status, if you want to track how the server itself is doing, Nagios is a great tool. For content monitoring, I'm not familiar with any specific open source applications, but a little time on sourceforge might turn up some options.
This was first published in May 2005