http://lpoaj76nfopd5lpinbskyqtroppamrzhhay3g4vvjm75st6ger34lbyd.onion/posts/2020/08/email-sucks.html
It needs two different kinds of servers (one for sending, and one for the clients to fetch new mail), whereas alternatives like Bitmessage are even serverless, or, like Matrix, require no more than one server per node. Security SMTP is not authenticated by default.