On Tue, 21 Jun 2005, John Horne wrote: > Curious about the last statement there. The manual says > 'use_postmaster' is for recipient callouts (callforward) only. Yet, > surely use of rfc-ignorant.org is for sender callouts (callback)?
One should not be using non-null envelope sender addresses on this kind of transaction to an arbitrary MTA. That risks creating mail loops - and just imagine what happens when they decide to verify your envelope sender address by calling _you_ out, using a non-null envelope sender, and you then call that envelope sender out to check it, and so on... If the remote host repudiates a transaction that has a null envelope sender, then it's a matter of policy what you then decide to do about it. Giving a 5xx response to a null envelope sender seems to be a routine with Chinese mail servers, and I happily put those into our callouts list so that they can reject their own mail, most of which is spam, and I'll have an RFC to throw at the tiny fraction that's bona fide. But a few other cases, we have to accept their mail for business reasons, even though they are violating the RFCs. In the call-forward situation, on the other hand, the normal arrangement is that you have some kind of relationship with the end MTA, so you can liaise with them to avoid the kind of unpleasantness that might arise with other, arbitrary, MTAs. As I see it, that's the key difference between sender callouts and recipient call-forwards. Just to make this clear: we don't use callouts on a blanket basis by any means! But it can, currently, be a useful strategy in limited circumstances against spam. Previous discussion of the topic applies, I don't want to open old wounds again. Apropos misbehaved MTAs, can someone explain the remarkable proportion of .br MTAs which seem to respond to unknown user with a 4xx response code? Some of that Brazilian spam gets retried for days on end, and repeatedly tempfailed with unknown sender, before they finally give up. -- ## List details at http://www.exim.org/mailman/listinfo/exim-users ## Exim details at http://www.exim.org/ ## Please use the Wiki with this list - http://www.exim.org/eximwiki/
