[exim] Log recipient with DKIM failures

Hello,

Sometimes I receive messages that fail the DKIM test and are thus
rejected. Sometimes, the sender complains about that (like mailing lists
being annoyed about bounced messages that they broke) or the recipient
misses a message and asks me where it went. Then I'll have to dig deep
into the Exim log to find the message by its timestamp because that's
all I have.

The log lines look like this:

> 2021-04-29 22:58:46 +0200 1lcDkD-00DLFl-S2 DKIM: validation error: Public key signature verification has failed.
> 2021-04-29 22:58:46 +0200 1lcDkD-00DLFl-S2 DKIM test failed: signature_incorrect
> 2021-04-29 22:58:46 +0200 1lcDkD-00DLFl-S2 H=pb1.pair.com (lists.php.net) [76.75.200.58] rejected DKIM : DKIM test failed: signature_incorrect

There's nothing about the sender or recipient in the main log. The
reject log doesn't contain that message at all. But in order to give the
mailbox owner some information about the rejected message, I have to
know the recipient of the rejected message.

What can I add to my Exim configuration to include that data in the
logs? The best option would be to include such events in the reject log
because that's already my main source for that and it has all the
details like sender and subject.

-Yves