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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:

• When the error count reaches $ smtpd_soft_error_limit (default: 10), the Postfix smtpd(8) server delays all non-error and error responses by $ smtpd_error_sleep_time seconds (default: 1 second).

• When the error count reaches $ smtpd_hard_error_limit (default: 20) the Postfix smtpd(8) server breaks the connection.

Postfix version 2.0 and earlier:

• When the error count is less than $ smtpd_soft_error_limit (default: 10) the Postfix smtpd(8) server delays all error replies by $ smtpd_error_sleep_time (1 second with Postfix 2.0, 5 seconds with Postfix 1.1 and earlier).

• When the error count reaches $ smtpd_soft_error_limit, the Postfix smtpd(8) server delays all responses by "error count" seconds or $ smtpd_error_sleep_time, whichever is more.

• When the error count reaches $ smtpd_hard_error_limit (default: 20) the Postfix smtpd(8) server breaks the connection.