Re: [exim] sender callout mail_from change

* on the Wed, Jan 31, 2007 at 09:45:59AM +0000, Peter Bowyer wrote:

>> No one has mentioned why sender callouts without a null sender are "bad"
>> yet. As far as I can see the worse that can happen is, a remote mail
>> server connects to yours, and sends a "MAIL FROM" and a "RCPT TO". You
>> then connect to the MX for the domain in the MAIL FROM, and do the same,
>> using the value of the "RCPT TO" in the mail from of the callout. They
>> then connect back to you to do a sender callout themselves. Then it
>> stops due to the cache... And this would only happen in the rare
>> circumstances that both servers are using sender callouts...
> If your server is performing a sender callout, it's because the sender
> isn't in its cache. When the reverse callout comes back, the sender is
> the same and still isn't in the cache because the first callout hasn't
> completed, so the loop continues.

Crap. Of course.

This only happens in the circumstances where both servers are using
callouts without null senders though. I guess that's why I've not seen
it yet. I need to rethink this now.

Mike