This sounds a lot like the cutthough concept - see spec.txt chapter 44.22.
I have only heard of this being used when forwarding the message to
nearby friendly mail servers, and I see that you cannot use DKIM and
cutthough.
I noticed in my log files that I have situations where some mail is received
from externally which is supposed to be forwarded to externally (with the
'redirect' driver, or 'remote_smtp' transport?). Now if my spam filter didn't
detect the message but it was rejected by the forward recipient, Exim will
generate a bounce message and try to send it to an arbitrary sender which is
mostly fake. This is causing backscatter, which may cause issues with my mail
server reputation in general. (I know that the RFC requires that behaviour,
but that RFC is broken and not practical, for well-known reasons.) The same
probably happens when the remote mailbox is full.
I tried to understand what the 'errors_to' directive does but it's hard to
follow those descriptions. So I'm not sure whether it's useful in my case.
So I had another idea. If queuing and bouncing is the problem, can I get rid
of that? If a message comes in from an unauthenticated sender, that must be
another mail server. Those have got time, there's nobody waiting in front of
a screen for the message to be submitted. So could Exim just try to deliver
the message immediately in this case? If that fails, it could directly reject
the message in the waiting original connection. No need for bounces.
If the remote error is permanent, so should be Exim's. Same for temporary
errors. Passing back the remote error message is probably a privacy concern
as it might disclose the forward address or provider.
This must only apply to forwards for unauthenticated senders. Authenticated
senders are my users, they provided a password and I know I can send them
bounces safely. And they're waiting for completion of the submission.
How would a configuration of this look like? Or where in the documentation
can I find more information about that?
-Yves
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Andrew C. Aitchison Kendal, UK
[email protected]
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