If your product is real, and it depends on interacting with external SMTP servers, then you should be testing it with real SMTP servers, like Exchange, sendmail, qmail, postfix, exim. Yes you could write something in Perl but why not use the real thing and do a real test.

This will allow us to verify ...
I would think that you need to verify that your app is constructing messages in a compliant fashion and transacting SMTP properly when pointed at differently configured daemons.