CSMail is a mail delivery framework intended to make it easy for applications to send e-mail without tying themselves to a specific mail client.
It supports plug-in back-ends; currently there are three back-ends in the distribution: Entourage, Eudora and NSMailDelivery (Mail.app).
The interface is very similar to that provided by NSMailDelivery; all back-ends support attachments, and currently both Entourage and NSMailDelivery support formatting (bold, italic, font changes, etc.)
You can find it at http://www.coriolis-systems.com/opensource/