A major aspect of any outgoing mail system is mime support IMHO. At least choosing between text/plain and text/html when sending should be possible. Some people may argue it should support multipart/alternative as well - I don't really think it is necessary (the current acs-mail package supports it, acs-mail-lite afaik doesn't). Attachments of any mime type should be possible.
What I think is important too is the character encoding of mails - I don't know how widespread utf-8 capable mail clients are. If they are rare it would be nice to be able to encode the mail in the recipient users' preferred encoding.
Representing hierarchy with References and In-Reply-To headers - some way of tracking message_ids so that we can send out mails that contain information about their parent messages, so that they find their correct place in the threaded view of mail programs that read those headers. Typical example are forum threads.
