Just like any other SMTP / POP3 server, plus a number of extended ad-hoc features for tor network use. With this mailserver you can send and receive emails from the internet and from the Tor network, boosting your privacy and protecting your metadata.
Using OnionMail, connections are always encrypted.
Transition servers do not store any info. Only the recipient's server saves the messages.
This type of server allows using emails inside the Tor network but without losing the opportunity to comunicate with common internet network.
Spam has a short lifespan because of custom blacklists.
Therefore every user can set his custom spam filters. Lastly, all servers are federated so as to enable server's SSL certificates verification.