Why deliver mail over Tor?
Even if you use TLS for your connections they are opportunistic. Even if you use OpenPGP for your connections, it is relatively easy for someone passively monitoring email traffic to correlate interesting metadata: who is communicating with whom, when and how much. Worse, it is trivial for a third party to know that two people are communicating.
By joining our little caramelized onion party, we can deliver mail over Tor onion services, and we can prevent this unauthorized privacy violation.