Overkill Interbuzz aggressively publishes stale DKIM keys in order to provide users plausible deniability of any leaked or published email. Append the domain which you are interested in to this URL and you will be provided a list of previously-active DKIM keys by selector.

These keys can be trivially loaded into OpenDKIM or any other DKIM signing software to forge DKIM-signed mails. You can then upload such forged mails to your email provider over IMAP so that they appear naturally at your providers' webmail site.