[Exim] 'verify' doesn't work in Exim 4.04 - why?

I have an ACL like this in Exim 4.04 configure file:

---------------------------------------------
begin acl

check_recipient:

accept hosts = :
accept recipients = postmaster

require verify = sender/callout

  accept sender_domains = +local_domains
         endpass
         message       = unknown user
         verify        = sender/callout

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

deny message = Relay not permitted

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and everything works fine except for the 'verify' commands, which don't
verify at all. I have no idea why :(

Robert