[Exim] callout suggestion

The following would be useful, I think

If exim could be configured with an alternate MAIL FROM address to use,
if "MAIL FROM: <>" wasnt accepted. (Yes, I know those sites are
blatantly wrong, but as an ISP we dont want to get too many people
whining about not getting their mail.

Someone here made a patch where it always used a different address, but
I'd prefer to use <> if its accepted, but it would be nice if it could
be made conditional

Something like "smtp_callout_alt_mailfrom"

If it was unset, then exim would use only <>, and fail the verify if
that wasnt accepted. If it was set, it would try <>, and if that wasnt
accepted, then try the value of that setting. Default would be unset..