Net::SMTP::Server is probably best for development testing, so you don't have to have a full blown SMTP server to test your email sending features.

I could be WAY off, but i'm not even sure it is a multithreaded server, and if that's the case, it makes it a very poor candidate for any kind of non-development use.

Also, as JavaFan stated, you can use a MTA (mail transport agent) such as sendmail that will take care of the actual mail sending for you.