Author: John W. Baxter Date: To: exim users Subject: Re: [exim] sender callout mail_from change
On 1/31/07 1:38 AM, "Mike Cardwell" <exim-users@???> 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...
Or you call out to a server that does greylisting 4xx responses at RCPT time
for most MAIL_FROM values, but at DATA time for <> (and the few obvious
other) MAIL_FROM values expecting that the calling-out server will never get
to DATA. So your callout is greylisted, and ... (what do you do?). (You
treat 4xx as success and all is well, or you treat it as callout failure,
then at best, they try again, your callout is accepted, and all is well
except that you have imposed a greylisting delay even though you don't do
greylisting.)
