Rated 5 out of 5 stars
I just ran into the same issue as reported by Martin in his review: https://addons.mozilla.org/EN-uS/thunderbird/addon/dkim-verifier/reviews/701478/
I am running DKIM Verifiier 1.3.6, Thunderbird 38.4.0, Fedora 22. Unchecking General:DNS:Get DNS servers from OS configuration fixed it. (Thank you Martin!)
Possibly related, my resolv.conf is constantly being changed by connecting/disconnecting to VPNs.
Thank you Phillippe for this add-on, it should be part of Thunderbird by default!