Re: [Exim] callout suggestion

On Fri, 7 Jun 2002, Philip Hazel wrote:

> On Thu, 6 Jun 2002, Dave C. wrote:
>
> > If exim could be configured with an alternate MAIL FROM address to use,
> > if "MAIL FROM: <>" wasnt accepted. (Yes, I know those sites are
> > blatantly wrong, but as an ISP we dont want to get too many people
> > whining about not getting their mail.
>
> How does this help? You will verify that you can reach the sender with a
> non-<> MAIL FROM, but if the message you now accept bounces, Exim will
> try to send the bounce with MAIL FROM:<>, so you still get a stuck
> bounce, which is what you are trying to avoid in the first place....

Hrm.. I hadnt thought of that, but my main concern was not stuck
bounces, but rejecting mail from addresses that were compleletly invalid
(mainly spam), and allowing through legitimate non-spam mail, even from
domains whose administrators/mailservers were unclued about null SMTP
envelope senders. I also validate RCPT TO addresses at SMTP time, so if
I accept the message, it has a valid addressee and I'm not going to have
to bounce it. (And I control the secondary MX too, so thats not a
concern)

If

MAIL FROM: <>
RCPT TO: sender_address@a_domain

fails at the MAIL FROM, but

MAIL FROM: postmaster@mydomain
RCPT TO: sender_address@a_domain

or

MAIL FROM: sender_address@a_domain
RCPT TO: sender_address@a_domain

succeeds, then it is less likely that the message that I am considering
acceptance of is spam (Its been my experience that spammers more often
forge addresses at large/popular mail domains that properly accept the
null sender) than if any/all of them failed at the RCPT TO:

Basically a:

"If the MX for the sending domain rejects MAIL FROM: <>, we wont accept
the mail at all"

policy is a little bit too hard-line for our customers to accept (even
if it is the RFC-compliant way), but I could get away with a policy of:

"If the MX for the sender domain rejects a RCPT TO: for the sending
address, regardless of wether the MAIL FROM: was <>, or
postmaster@ourdomain, we wont accept the mail"

and still block lots of spam...