[Exim] verify & MySQL

I've just updated to 4.x but have found that the config section:

========
  accept  domains       = +local_domains
          endpass
          message       = unknown user
          verify        = recipient
========

Does not not work correctly; just how does Exim 4.x verify the recipent
address? ANd how do I get it to verify it via info from the database
instead?