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 127.0.1.1
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.
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 email@example.com.