[Exim] MAIL ACL and sender verification

Hello,

I've just started playing around with the new 4.12. In particular, I was
looking forward to the new MAIL ACL, so that I can reject invalid (as in
un-verifiable) senders at MAIL time (to help in providing
easier-to-understand messages to end users).

However, it seems I can't do this:

acl_my_mail_acl:
require verify=sender
accept

(or variants thereof)

Exim says: "cannot test verify in MAIL ACL"

Am I doing something silly, or is it simply not possible to do
verifications in the MAIL ACL? What's the reasoning behind this?

Thanks,

Tim