Is Nagios good for monitoring e-mail?

I guess this depends on what you mean by e-mail monitoring. If you mean monitoring the status of an e-mail server, the Nagios is definitely a good choice. You can easily track file size, resource usage, etc. In terms of monitoring account sizes, etc., there are either existing plugins or you could easily write one yourself.

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.