One of the greatest improvements in the 2.7 tree is the ability to defer transmission of received messages to a SMTP proxy until the message receiption completes (smtpd_proxy_options = speed_adjust).
However, compared to the content_filter setting, it stil lacks some functionality: A content filter supports all (well, the ones I'm aware of) the settings which are valid in a transport(5) map. It thereby provides a very easy way to realize DNS based kiad balancing by specifying a content filter setting like "filter:mx-filter.example.com:10024", where the zone "mx-filter.example.com" holds several MX entries pointing to the content filter machines. As far as my understanding goes that is possible because the content_filter setting merely overrides the nexthop setting, making all standard functionality (MX lookups, queuing etc.) available. So, my question is: Are there any plans to further enhance Postfix's functionality as far as SMTP proxy filtering is concerned, and make that kind of load balancing available to pre-queue setups using smtpd_proxy_filter? Of course, I realize that a proxy setup is much more time critical than a content_filter setup. One would have to set a low timeout for trying any of the proxy filters, and one would perhaps need a way to specifiy different timeouts for each try (e.g., specify a short timeout for the first proxy filter that is contacted and a long timeout for the last one - better get a message filtered at all than to discard it). Still, getting load balancers out of a mail setup would be a nice thing. Stefan
