[Exim] Sender verification...

I have noticed that a large proportion of all the spam messages we are
getting have an invalid sender address, so I've considered enabling the
sender address verification of Exim. It does, however, seem a bit drastic
to plainly reject all messages that can't be verified. Also, the
alternative of adding failure messages to the log files doesn't seem to
helpful. Is some other kind of verification failure notification, like
updated headers, available?
--
Toralf Lund <toralf@???>