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sendmail Performance Tuning by Nick Christenson

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3.1. What Happens During Relaying

Consider a high-level view of what happens on a sendmail system that acts as an email gateway. First, an external system opens an SMTP connection with the gateway. The message is sent to the gateway, which writes it to local storage and then acknowledges (and accepts responsibility for) the message. At that point, the sending machine closes the SMTP connection and can delete its own copy of the message. The gateway next opens an SMTP connection to the message’s true destination (or perhaps the next hop). It transmits the message to this machine, waiting to hear that the remote machine has accepted the message. At that point, the gateway may close its connection and remove the message from its queue.

Of course, ...

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