Well, I've tried various MTA's and ultimately came back to good old sendmail. I've used Exim, but any wish, like virus scanning or whatever, requires upgrading Exim, or it is not possible at all. I've used Postfix, but all functionality is hard-wired and it fails if you get non-standard wishes.
Sendmail performs quite well, can do anything, is always supported, and is rightfully blamed and flamed for its configuration file. Sendmail has a spartan sendmail.mc file for it's configuration. It ain't modern, but it is not hard to modify either and oh boy it is flexible.
