List of email providers (categorized by tor/privacy friendliness).
Note: This list does not say anything about the trustworthiness of the provider itself.
http://sofia.ecnetworld.com <--> http://e3qixnonjb76zovy.onion (great provider. You should try it. Supporting multiple identities and send emails with fake identities) - (requires subscription payment)
http://maillvohhojeoliq.onion <--> Great Anonymous Mail Service, always from the ECNET Staff (the same of Sofia Mail System) with multiple networks access, POP3/IMAP free service (required a small donation, no minimal request)
http://xdkriz6cn2avvcr2vks5lvvtmfojz2ohjzj4fhyuka55mvljeso2ztqd.onion/ Cock.li (Very transparent provider, no personal information collected at sign up. Has an onion service and Tor-facing webmail. Requires JS, but uses the open-source RoundCube software. POP3/IMAP support.)
https://mailbox.org (30 days trial for free; see how to use their Tor exit or their mail server Onion site)
https://scryptmail.com (7 days trial for free; webmail only, no POP3/IMAP)
https://riseup.net (registration is limited to invite codes by fellow activists)
https://mail.ru (secondary email is not verified, BAD will lock your account later when using tor, no anon recovery possible; beware: mail.ru develops and spreads spyware called mail.ru guard)
Mail2Tor: http://mail2tor2zyjdctd.onion/ (not very reliable: Delayed Mail (still being retried), lost connection with 127.0.0.1)
http://torbox3uiot6wchz.onion/ (There is no connection between TorBox and the public internet: All the messages are sent and received within TorBox).
https://protonmail.com (webmail only, no POP3/IMAP, requires verification with other email account)
Unseen.is - https://www.unseen.is/ (requires secondary e-mail address).
fastmail.com (requires SMS verification)
tutanota.com (locks accounts created via tor: https://lists.torproject.org/pipermail/tor-talk/2016-May/040867.html )
Lelantos-Project: lelantoss7bcnwbv.onion (requires subscription payment)
openmailbox.org (blocks tor users from creating accounts: Unable to continue. Please contact the administration team (internal error code: 10))