CiviMail DKIM signature failure
We did experience that for specific mail providers. Local delivery and most mail providers were fine or not having any non 7bit encoded characters.

We ended up forcing a downconversion to 7bit encoding on our MTA prior to signing with DKIM as described here for postfix: http://postfix.1071664.n5.nabble.com/On-DKIM-and-Content-Transfer-Encoding-tp68767p68769.html