If you were to log in, you'd be able to get more information on your fellow community member.
I've had some good experiences with Exim (see The Exim home page for more information). Exim is an open-source mailer, adopted by the GNU project, developed by Philip Hazel, at Cambridge University in the UK.
It allows seperate aliases files for each domain you are hosting - for example I ensure that firstname.lastname@example.org and email@example.com go to two separate addresses, while hosted on the same box.
I don't currently have a patch available for the "multiple addresses for MTA's" problem, and I'm not sure how simple this is for the MTA to control. I imagine that what is required is a patch to make the server bind to a specific address, rather than just the first one in the list. I've seen this type of patch for some IRC clients, but not for an MTA.
I use exim in a busy ISP - it's easy to manage, has a useful API to hook custom tools into, and seems to perform well. I've had no problems hooking in listservers such as majordomo in...