Here you'll find the DKIM private keys that were previously used for signing emails on the mail server.

Publishing these keys restores plausible deniability. As soon as the private key is published, you can no longer assert that an email signature is authentic; anyone could have faked it.

This article on publishing dkim keys gives a good overview of why this is a good idea.

The name of the key file roughly indicates when the key was in use.