The Simple Mail Transfer Protocol is an internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. Timeouts are an essential feature of an SMTP implementation. If the timeouts are too long (or worse, there are no timeouts), Internet communication failures or software bugs in receiver-SMTP programs can tie up SMTP processes indefinitely. If the timeouts are too short, resources are wasted with attempts that time out part way through message delivery.
TIMEOUT SETTINGS
Greeting : A Sender-SMTP process needs to distinguish between a failed TCP connection and a delay in receiving the initial 220-greeting message. Many receiver-SMTPs accept a TCP connection but delay delivery of the 220 messages until their system load permits more mail to be processed.
Mail : It is used to establish the return address. By default, it is 300 seconds.
Recipient : A longer timeout is required if the processing of mailing lists and aliases was not deferred until after the message was accepted.
Data Initialization : This is while awaiting the "354 Start Input" reply to a DATA command.
Data Block : This is while awaiting the completion of each TCP SEND call transmitting a chunk of data.
Data Term : 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 is a waste since the message has been successfully sent.
Session : After this time, the session is expired. By default, it is 600 seconds.
Runtime Changeable No