QUIT command

What it means

A client sends QUIT when it has nothing more to send, and the server must answer 221 and close. RFC 5321 does not let a server close a connection on its own in normal operation except after QUIT, after a 421 reply, or after a timeout, and says a server being shut down should send 421 before it goes.

When a connection is lost without either, the client should treat the open transaction as if it had received a 451 temporary failure and try again later. Messages already accepted with 250 before the connection ended remain the receiving server's responsibility, so a dropped connection loses nothing that was accepted.