On 2010-12-09 03:56, W B Hacker wrote:
> IOW: rejection=SOMETIMES, (post-session) bounce=NEVER
Recipient callouts can't help for the case of data-time reject
(commonly dues to content-scan for virus/spam).
I've often wanted to get the tuits together for a synchronous-delivery
mode switch in Exim - a control modifier for ACL which converts
the existing source session by placing an immediate destination
recipient callout and then hooks the pair of datastreams together.
The intermediate Exim would, apart from copying data and responses,
only monitor for the termination of data phase. The point of
doing this is to get the ultimate recipient system's response back
to the sender, synchronously, avoiding another class of bounces.
The control would typically be used in the RCPT or PREDATA ACL.
If we needed to be able to code it in earlier ACLs the implementation
would have to delay action until RCPT time. If we needed to handle
it at DATA (or MIME) time then we would play out the already-written
spool file (note that the RCPT-time usage avoids disk I/O, a minor benefit).
I don't see any use in QUIT, NOTQUIT, EXPN, VRFY, ETRN. I've not
thought hard about not-smtp, but it would probably work like RCPT.
A fallback to traditional store-and-forward mode should be available
if the target system is unavailable. Bounces would still be generated
for this case as per current handling; we're only addressing the use
of MX for "user moved on" forwarding, not "resilience versus intermittent
connectivity".
Cheers,
Jeremy
