At 15:59 -0400 on 08/28/2011, Hector Santos wrote about The purpose of QUIT:
In the 1982, but still currently the official SMTP standard, STD10
(RFC821) has QUIT as a SHOULD. In the 2000 document RFC2821 and the
newer (2009) RFC5321, it was made into a MUST.
I would like to hear what was the (DRUMS) thinking towards making it a MUST.
While I do not know DRUMS reason, I can think of one reason to
require the QUIT. It signals that the client is done and has received
all of its mail and has logged all of them. If I remember correctly
the deletes of mail sent from the server to the client un response to
the client's DEL commands is performed when the QUIT is received by
the Server not as each DEL is received. If the connection drops
before that, the deletes will not occur and the messages will be
offered again at the next connection.