I've got Thunderbird 52.2.1 on OSX, and I've seen something odd, but apparently consistent, with the way the DKIM Verifier contacts DNS after a laptop sleep.

The problem seems to arise when I have TB open on one network, then sleep the laptop, and then wake up on a different network such that resolv.conf is reset by DHCP, all the while having TB still running.

Whilst other TB functions work Ok - I can still reconnect to the IMAP servers post wakeup - the DKIM Verifier seems to end up in a condition where it claims that that no DNS Servers are alive.

A restart of Thunderbird fixes everything of course.

I've sniffed the network traffic as the Verifier attempts to perform its DMARC lookups, and as far as I can see it doesn't make any attempt to contact any DNS Server - not even the one that was previously set in resolv.conf.

The TB Error Console with debugging enabled shows items similar to below for each DKIM Verification attempt:

2017-06-28 10:06:14 DKIM_Verifier.JSDNS DEBUG no DNS Server alive
2017-06-28 10:06:14 DKIM_Verifier.DNSWrapper DEBUG result: ({data:null, rcode:2, secure:false, bogus:false})
2017-06-28 10:06:14 DKIM_Verifier.DMARC ERROR DKIM_DNSERROR_SERVER_ERROR: DKIM verifier internal error: Error connecting to the DNS server (resource://dkim_verifier/helper.jsm:10:32) JS Stack trace: getDMARCRecord/promise<@dkimDMARC.jsm:282:10
Log.jsm:753
2017-06-28 10:06:14 DKIM_Verifier.Policy DEBUG shouldBeSigned: false; sdid: ; hideFail: false; foundRule: false
2017-06-28 10:06:14 DKIM_Verifier.AuthVerifier DEBUG authResult: ({version:"2.1", dkim:[{version:"2.0", result:"none", res_num:40, result_str:"No Signature"}]})