http://6sxoyfb3h2nvok2d.onion/tor/EximAndTorHowto describes how to use exim to send mail to .onion addresses. I've written a preliminary whitepaper that discusses issues surrounding this.

Please see the FAQ first, then download the code. Note that it requires exim4.

Notes If you have to risk that your DNS lookups are seen by someone else, make sure you don't look up stuff when getting an incoming torified connection!