Re: [exim] sender callout mail_from change

* on the Mon, Jan 29, 2007 at 06:40:01PM +0000, Mike Cardwell wrote:

>>>> Recently, when I've re-added a 'require verify =
>>>> sender/callout=5s,maxwait=30s,fullpostmaster,defer_ok' to my ACL, I
>>>> noticed that some of the mails are rejected because the sender's mail
>>>> server refuses to accept mail with <> as origination.
>>>> Is there a way to make the MAIL FROM:<> to be replaced with some valid
>>>> address? I was thinking about using something like MAIL
>>>> FROM:<verifycallout@???> where verifycallout as an alias to
>>> Yes. Add ,mailfrom=verifycallout@$domain to the verify condition.
>> ... and watch out for loops.
> Hi. Just for my own piece of mind, what loops could occur here? Don't
> forget the callout cache...

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...

What part of the puzzle am I missing?

Mike