4.5.3.2 Timeouts An SMTP client MUST provide a timeout mechanism. It MUST use per- command timeouts rather than somehow trying to time the entire mail transaction. Timeouts SHOULD be easily reconfigurable, preferably without recompiling the SMTP code. To implement this, a timer is set for each SMTP command and for each buffer of the data transfer. The latter means that the overall timeout is inherently proportional to the size of the message. Based on extensive experience with busy mail-relay hosts, the minimum per-command timeout values SHOULD be as follows: Initial 220 Message: 5 minutes An SMTP client process needs to distinguish between a failed TCP connection and a delay in receiving the initial 220 greeting message. Many SMTP servers accept a TCP connection but delay delivery of the 220 message until their system load permits more mail to be processed. MAIL Command: 5 minutes RCPT Command: 5 minutes A longer timeout is required if processing of mailing lists and aliases is not deferred until after the message was accepted. DATA Initiation: 2 minutes This is while awaiting the "354 Start Input" reply to a DATA command. Data Block: 3 minutes This is while awaiting the completion of each TCP SEND call transmitting a chunk of data. DATA Termination: 10 minutes. This is while awaiting the "250 OK" reply. When the receiver gets the final period terminating the message data, it typically performs processing to deliver the message to a user mailbox. A spurious timeout at this point would be very wasteful and would typically result in delivery of multiple copies of the message, since it has been successfully sent and the server has accepted responsibility for delivery. See section 6.1 for additional discussion. An SMTP server SHOULD have a timeout of at least 5 minutes while it is awaiting the next command from the sender.
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