Smail seems to be the GNU project's official MTA. Its sole source of Fun is to take E-mail messages and pass them along, either to other relays or perharps to users, depending on destination.

I used smail for some time (now I use PostFix); it is not as flexible as Sendmail, but at least it was reasonably easy to configure (with Debian's configurators), and it did fine in smart host/dial-up use. I'm not sure how it compares to other MTAs in "heavy use".

Smail used to be Debian's default MTA, but 2.2 uses Exim.