Well, as I see it, Net::SMTP is exactly what the name implies: an easy-to use client for the SMTP protocol. Nothing more, nothing less. It's pretty lowlevel stuff, and if I was writing an application requiring robust mail handling, I'd probably go with a module that can guarantee that a mail will eventually reach its destination and won't choke on bad recipient addresses, just as I wouldn't use Assembler to do regular expression handling. On the other hand, if I was writing my own mail transport module it's possible that I'd base it on Net::SMTP. It's just a question of chosing the right tools for the job.

However, Net::SMTP has a (albeit small) edge on the modules you mentioned: it's included in the base distribution, meaning easier script installation for uneducated admins.

Cheers,
--Moodster