Is there a Python MTA (Mail transfer agent)

Yes, Twisted includes a framework for building SMTP servers. There's a simple Twisted-based email server available here (also see here for some information about its development).

If you want something closer to a mail application server, there's Lamson.

If you're looking for a full MTA solution you should check out http://slimta.org/ or as previously mentioned here http://lamsonproject.org I myself has experimented a bit with slimta and it seems to work well.

It's pretty new, so nothing like the maturity of Twisted's SMTP, but there's also Lamson.