The client has responsibility for delivering a message to one or more envelope recipients, or notifying an envelope sender if delivery is not made. It wants to transfer this responsibility to the server.
The client waits for the server's greeting:
220 heaven.af.mil ESMTPIf the server does not accept the connection, the client quits.
The client then sends a HELO request, and waits for the response:
HELO sun.af.mil 250 heaven.af.milIf the server does not accept the request, the client quits.
The client then sends a MAIL request showing the envelope sender address, and waits for the response:
MAIL FROM:<firstname.lastname@example.org> 250 okIf the server does not accept the request, the client quits.
The client then sends one RCPT request for each envelope recipient address, waiting for a response after each address:
RCPT TO:<God@heaven.af.mil> 250 okIt keeps track of which addresses are accepted. If none of the addresses are accepted, the client quits.
The client then sends a DATA request, and waits for the response:
DATA 354 go aheadIf the server does not accept the request, the client quits.
The client then sends the encoded message, waits for the response, and quits:
Date: 8 Aug 1998 04:10:45 -0000 From: email@example.com To: God@heaven.af.mil Subject: chariot is back in service I patched up the wheel. It's as good as new. . 250 ok 902549473 qp 24035 QUIT 221 heaven.af.milIf the client encounters a message I/O error, it closes the connection immediately, without sending a final dot and without sending a QUIT request.
C H M R D E result -------------------- 5 deferral (some clients: failure) 4 deferral 2 5 deferral (some clients: failure) 2 4 deferral 2 2 5 failure (a few clients: deferral) 2 2 4 deferral 2 2 2 5 failure 2 2 2 4 deferral 2 2 2 2 5 failure 2 2 2 2 4 deferral 2 2 2 2 3 5 failure 2 2 2 2 3 4 deferral 2 2 2 2 3 2 success
In particular, say the client sends the dot at the end of the encoded message, the server accepts the message, and the connection closes before the client receives the server's response. The client does not know that the server accepted the message; it will try again later. Both the client and the server now have responsibility for the delivery. The recipient will end up receiving two copies of the message.
To minimize the chance of this event, high-quality servers respond as quickly as possible to encoded messages.
If the second message has the same contents and the same envelope sender as the first message, the client can save time by merging the recipient lists and sending the message just once. Exception: if the new recipient list is longer than the server can handle, the server will end up deferring some recipients.
I recommend that clients avoid holding connections open when they do not have messages to send immediately. An idle connection chews up memory on the server that could be used for other tasks.