Note: this feature is not included with Postfix version 2.1.
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:
anvil(8) breaks, then connection limits stop working).
IMPORTANT: These limits are designed to protect the
against flagrant abuse. Do not use these limits to regulate legitimate
traffic: mail will suffer grotesque delays if you do so.