Check MX record hostname validity #672

Job	Run time
docker-build	41s
test	6m 36s
build (linux, amd64)	37s
lint	2m 19s
build (linux, 386)	40s
build (linux, arm)	36s
build (linux, arm64)	35s
build (darwin, amd64)	42s
build (darwin, arm64)	32s
build (windows, amd64)	33s
build (windows, 386)	33s
build (windows, arm)	44s
build (windows, arm64)	39s
integration-test-serverside (f0023141ec429d912e329665f937d05b178a3fee)	2m 2s
integration-test-clientside (v0.13.2)	2m 13s
analyze	2m 5s
	22m 7s

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