On Thu, 2010-01-07 at 14:50 +0100, Paul Sheer wrote: > I expect this could create a recursive loop if two mail servers that both > implement this feature send mail between one another. How is a loop avoided?
No, because if done sanely the callout is always done with an _empty_ sender, as if it were going to send a bounce. So there's no way it can trigger a loop. > Are there any blogs, caveates, discussions about this kind of behaviour in > general? Some people whine that it's not sustainable or that it leads to a DDoS, but that's mostly nonsense -- I'm not aware of any case where callouts have actually lead to such a thing, and in any case the amount of resource it takes to handle a callout is _tiny_ in comparison with what a modern mailserver has to do to process incoming spam anyway. The main reason for not doing callouts because they have "false" positives -- there are a surprising number of idiots out there who send mail from an address which can't receive bounces, and thus fails sender verification. -- dwmw2 -- ## List details at http://lists.exim.org/mailman/listinfo/exim-dev Exim details at http://www.exim.org/ ##
