postfix-2.7-20100117 changes the meaning of content filters of the
form "transport:" (note: no next-hop destination) so that this
form can be used to implement sender reputation schemes. Typically,
mail is split into classes, and all mail in class X is sent out
from an SMTP client IP address that is reserved for class X.
According to discussions on the Postfix list there is a legitimate
need for such functionality. Until now this requires one Postfix
instance per source IP address.
The change introduces one minor incompatibility. As always, there
is a compatibility option to restore the old behavior. This option
is not needed with SMTP-based content filters, because these always
have an explicit next-hop destination.
With pipe-based content filters that specify no next-hop destination,
the compatibility option restores the order of deliveries when a
filter is congested (it restores from per-destination round-robin
selection back to approximate FIFO selection).
More details in the release notes quotes below.
Wietse
Major changes with snapshot 20100117
====================================
The FILTER action in access maps or header/body_checks now supports
sender reputation schemes that dynamically choose the SMTP source
IP address.
This is implemented by specifying FILTER actions with empty next-hop
destinations in access maps or header/body_checks, and by configuring
in master.cf one Postfix SMTP client for each SMTP source IP address,
where each client has its own "-o myhostname" and "-o smtp_bind_address"
settings.
Incompatibility with snapshot 20100117
======================================
The meaning of an empty content filter next-hop destination has
changed. Postfix now uses the recipient domain, instead of using
$myhostname as in Postfix 2.6 and earlier. To get the old behavior
use "default_filter_nexthop = $myhostname", or specify a non-empty
next-hop content filter destination.
