[Exim] Callout verification

I'm aware this has been discussed on the list in the past, but any
references have either dissipated without a definitive outcome or are
several years old.

Many mailservers are configured (against the RFC guidelines) to refuse
mail from "<>". When performing a sender verify callout from exim,
exim presents "MAIL FROM:<>", and if the remote mailserver rejects
this, it would mean the message being verified is rejected - without
even getting to the "RCPT TO:" stage.

It seems to myself it would be preferable (though there may be good
reasons against this, please educate me) for the verify to only fail
if the 5xx error code is generated after the "RCPT TO:", since
after all this is the part we're trying to verify - not the
mailservers configuration to the RFC standards.

Other discussion I see suggests the ability to change the "MAIL FROM:"
address presented in the callout, this would also be useful though
obviously there are drawbacks such as cascading callbacks and loops,
but nevertheless, the option to configure this should you wish would
be very desirable.

Primarily I'd like any discussion to focus around my point in
paragraph two, regarding the point at which the 5xx error is generated
and how the result of this is handled. I'm not sure if there are MTA's
which don't give the error regarding an invalid "MAIL TO:" address
until after the "RCPT TO:" is given? Perhaps someone knows if this is
the case, if it is then it does somewhat throw a spanner in the works
of my suggestion. In which case unless someone can see a way around
it, maybe the configurable callout "MAIL FROM:" address would be
desirable?

Thanks in advance.