[exim] odd DKIM verify failure

A rare piece of spam was able to get through my exim based defenses.
It was DKIM signed, and the log entry when it was received looks like
this:

2018-07-13 15:46:16 1fe6pM-0007WY-7X PDKIM: d=wallstreetinsider.org s=mail [failed key import]
2018-07-13 15:46:16 1fe6pM-0007WY-7X <= info@??? H=mail2.wallstreetinsider.org [139.9
9.102.117]:48086 P=esmtps X=TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256 CV=no S=3826 id=5uCA1Amsmymlao3vrJH
2X4YXb3UbMQekyO3UkbDZ4@localhost

Is there a way to have an ACL deny rule specifically for this failure
mode? Not necessarily in acl_smtp_dkim, maybe in a generic acl based on
the key not being available in DNS?

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