Check MX record hostname validity #644

Job	Run time
test	6m 34s
docker-build	41s
lint	1m 2s
build (linux, amd64)	35s
build (linux, 386)	32s
build (linux, arm)	33s
build (linux, arm64)	31s
build (darwin, amd64)	34s
build (darwin, arm64)	34s
build (windows, amd64)	40s
build (windows, 386)	42s
build (windows, arm)	40s
build (windows, arm64)	37s
analyze	1m 41s
integration-test-serverside (f0023141ec429d912e329665f937d05b178a3fee)	2m 21s
integration-test-clientside (v0.13.2)	2m 18s
	20m 35s

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