Measures against clients that make too many connections

Note: this feature is not included with Postfix version 2.1.

The Postfix smtpd(8) server can limit the number of simultaneous connections from the same SMTP client, as well as the number of connections that a client is allowed to make per unit time. These statistics are maintained by the anvil(8) server (translation: if anvil(8) breaks, then connection limits stop working).

IMPORTANT: These limits are designed to protect the smtpd(8) server against flagrant abuse. Do not use these limits to regulate legitimate traffic: mail will suffer grotesque delays if you do so.

• An SMTP client may make up to $ smtpd_client_connection_count_limit simultaneous connections (default: 50). This is half the default process limit.

• An SMTP client may make up to $ smtpd_client_message_rate_limit message delivery requests per unit time (default: no limit).

• An SMTP client may send up to $ smtpd_client_recipient_rate_limit recipient addresses per unit time (default: no limit).

• An SMTP client may make up to $ smtpd_client_connection_rate_limit connections per unit time (default: no limit).

• These limits are not applied to SMTP clients in the networks specified with $ smtpd_client_event_limit_exceptions (default: clients in $ mynetworks may make an unlimited number of connections).

• The anvil_rate_time_unit parameter specifies the time unit over which client connection rates are computed (default: 60s).