Mail with DKIM Signature gets T_DKIM_INVALID flag by SpamAssassin
Solution 1:
You could give this package a try instead: libmail-dkim-perl
One of the walk-throughs linked in the Spamassassin documentation points out a hint that I would recommend to follow:
"You should NEVER need to use CPAN on a RPM or DEB based Linux distribution."
Solution 2:
This happend because of the missing perl module Mail::DKIM. Without that, every mail with dkim signature will fail the verification. Even those with correct signature.
You can find futher information about it here.
A simple fix is to install the missing module via cpan.