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sendmail Performance Tuning by Nick Christenson

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4.2. Recipient Verification

Unless it has been configured not to do so, when sendmail is told that a message is bound for a recipient in a local domain, it will look up the user name to make sure that the recipient actually exists on the server before it attempts to deliver the message. This step allows a misaddressed message to be rejected during the portion of the SMTP dialogue where the envelope is exchanged. If a message can be rejected during the envelope exchange, the server won’t have to expend resources associated with processing and queueing the message itself. High-performance email servers often store very large numbers of individual mailboxes. Operating systems sometimes don’t perform very well, or even correctly, if the /etc/passwd ...

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