[Exim] Error message for DATA ACL

I am on exim-4.14.

I have been unable to get a suitable error message returned in the SMTP error when

verify = header_syntax and
verify = header_sender

cause a failure. All I can get is "Administrative prohibition". The rejectlog
contains a very comprehensive message such as "There is no valid sender in any
header line". I have tried using "message = $acl_verify_message" but that only
contains the default "Administrative prohibition" string.

How do I get the message which is put into rejectlog into the SMTP failure?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter