[SwellJoe]

You don't need to get clever here. Any SMTP server will deliver locally if you just give it a username.

So, any mail server will do. Just configure it to only accept mail from/for localhost. I know that Perl and Python both have good small SMTP servers available, and I imagine most dynamic languages would also (if it even matters what language it is in). So, you could use one of those, fire it up on a high port as part of your test process and send a message to the user running the test (which would then deliver to the users Maildir or whatever, based on configuration).

But I would just use Postfix. It's small, really easy to configure, and can be installed on most modern Linux distros with one command. It can easily be configured as I've suggested (set it to listen only on loopback, and set the destination to only localhost, and then send mail to "joe" rather than "[e-mail]").