[exim] denying email when sender verify fails

Hello,

on our auth-smtp Server we have the following acl:

acl_check_rcpt:

[...]

require verify = sender

[...]

accept  authenticated = *
deny    message       = relay not permitted

The problem is that even when the sender verify fails the email will be
accepted if the user has authenticated. What do I have to change so that
the email will not be accepted if the sender verify fails and how can I
can I change the error message returned to the client (I guess just a
mesdsage=... line)?

Regards
Marten