/etc/logcheck/ignore.d.server/dkfilter is in logcheck-database 1.3.17.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | ^\w{3} [ :0-9]{11} [._[:alnum:]-]+ dkfilter.in\[[0-9]+\]: DomainKeys verification - (pass|neutral)( \(no signature; (no policy for [._[:alnum:]-]+|domain testing)\))?; (from|sender)=<[^[:space:]]+>(, message-id=<[^[:space:]]+>)?$
^\w{3} [ :0-9]{11} [._[:alnum:]-]+ dkfilter.out\[[0-9]+\]: DomainKeys signing - (signed|skipped \(wrong sender domain\)); (from|sender)=<[^[:space:]]+>, message-id=<[^[:space:]]+>$
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