Props to RhetTBull. MIME::Lite is the ticket.

Spend some time learning about MIME types and pay close attention to the mail reader being used by your clients. AOL can't handle things like multipart/alternative mails. Yahoo isn't too good at it either. Some really paranoid mail readers such as SQWebmail won't let you view HTML mails. So if possible, keep your life simple and use plain old text for automated mails that are going out to the world. Eat chicken every day. I like chicken.