[exim] Forward from external immediately without queue

Hello,

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