Check MX record hostname validity #655

Job	Run time
docker-build	38s
lint	58s
test	4m 46s
build (linux, amd64)	33s
build (linux, 386)	42s
build (linux, arm)	37s
build (linux, arm64)	44s
build (darwin, amd64)	41s
build (darwin, arm64)	39s
build (windows, amd64)	38s
build (windows, 386)	33s
build (windows, arm)	35s
build (windows, arm64)	34s
integration-test-clientside (v0.13.2)	2m 1s
integration-test-serverside (f0023141ec429d912e329665f937d05b178a3fee)	2m 5s
analyze	2m 6s
	18m 50s

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