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.