The problems might be exacerbated when filtering or relaying software makes changes to a message. Without specific precaution implemented by the sender, the footer addition operated by most mailing lists and many central antivirus solutions will break the DKIM signature. A possible mitigation is to sign only designated number of bytes of the message body. It is indicated by l tag in DKIM-Signature header. Anything added beyond the specified length of the message body is not taken into account while calculating DKIM signature. This won't work for MIME messages.