[exim] recording failed rcpt to's

Hi All -

I am trying to record failed rcpt's in real-time (not from searching 
through log-files).   At the moment my rcpt to ACL says -
  accept  domains       = +local_domains
      endpass
       message       = unknown user
       verify        = recipient

I plan on inserting the information about the failed rcpt to into a db
(I am already using the MySQL driver with no problems....) - I am just
confused about how to put the command in the ACL (so it only executes if
the verification fails?

What would be the best method? Any suggestions/advice?


Cheers,
Andrew.