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Postfix Documentation
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Slowing down SMTP clients that make many errors

The Postfix smtpd(8) server maintains a per-session error count. The error count is reset when a message is transferred successfully, and is incremented when a client request is unrecognized or unimplemented, when a client request violates access restrictions, or when some other error happens.

As the per-session error count increases, the smtpd(8) server changes behavior and begins to insert delays into the responses. The idea is to slow down a run-away client in order to limit resource usage. The behavior is Postfix version dependent.

IMPORTANT: These delays slow down Postfix, too. When too much delay is configured, the number of simultaneous SMTP sessions will increase until it reaches the smtpd(8) server process limit, and new SMTP clients must wait until an smtpd(8) server process becomes available.

Postfix version 2.1 and later:

Postfix version 2.0 and earlier:

Postfix Documentation
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