These days where NDR's are becoming more and more harmful, it's very
useful to return as many errors as possible directly in the SMTP
session. Unfortunately not everyone has seen the light and put Exim on
their mail server, so it can often be handy to have Exim as an
incoming mail relay. This works nicely in general, but it often means
that the final server rejects mail which Exim accepted.
It would be very handy if Exim could try to deliver the mail (assuming
it passed all checks) before actually accepting the mail in the
incoming SMTP session. That way, errors like non-existent user would
be relayed immediately to the original sender, making NDR's
unnecessary.
There are various practical problems with this suggestion, but do they
make it useless in all cases?
