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
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: