This is a class that can send all sorts of email. Plain text, HTML (with or without images), attachments etc. First written in 2000, it's very mature and is in use by lots of major websites. Updated for PHP5, it takes advantage of PHP5 features to be very efficient at sending all sorts of email.
You can download everything by using the archive, or you can view and/or download the individual files using the links below. You'll find the documentation below.
Download the archive: Rmail.zip
The source files:
The documentation (which I'm continuously adding to) is here.
This is by no means a complete changelog, but a list of the more recent changes I've made to Rmail.