What are Tor based e-mail services?

I found:
http://mail2tor2zyjdctd.onion - Mail2Tor (mail2tor.com)
http://lelantoss7bcnwbv.onion - Lelantos (lelantos.org) not free
http://bitmailendavkbec.onion - BitMessage (bitmessage.ch)
http://sigaintevyh2rzvw.onion - SIGAINT (sigaint.org)