Rated 4 out of 5 stars

I had to uncheck the "Use OS DNS servers" setting (loosely translated from French) because on my Ubuntu 14.04 the local cache resolver "dnsmasq" does not answer, as I read in the console :

2015-02-05 18:40:21 DKIM_Verifier.JSDNS INFO Resolving 20120113._domainkey.gmail.com TXT by querying

2015-02-05 18:40:21 DKIM_Verifier.JSDNS DEBUG 20120113._domainkey.gmail.com/TXT: No answer, no authority to recurse on. DNS lookup failed.

Once I uncheck this, it works.

This review is for a previous version of the add-on (1.3.1). 

Unfortunately I am unable to reproduce this on my Ubuntu 14.04 installation. If you are interested in trying to find the problem please write an e-mail to dkim.verifier.addon@gmail.com.