It can’t be reliable by design. All it takes is dead server without backup in the middle between you and recipient. Reliable protocol requires acknowledges and

It can’t be reliable by design. All it takes is dead server without backup in the middle between you and recipient. Reliable protocol requires acknowledges and retransmissions at every step. There’s no such thing in email. It is reliable enough in real life with everything set up properly, that’s true. If mail going to spam can be counted as delivered.