You didn't say what the total number of emails being sent are. If it's small (in the hundreds), I would look at wrapping Mail::Mailer and a tuned sendmail configuration; however, if it's in the thousands, Mail::Bulkmail may be fine. Mail::Bulkmail hasn't been updated in a while but I just looked at the code last month and except for a few colorful comments by the author, it was still quite solid.

You may also want to look into some of the mailing list managers out there.

-derby