Revert "Some domains are listed as live even when DNS times out"

[jsf9k]

See comments by @h-m-f-t in #91

[jsf9k]

@h-m-f-t, the original impetus for #91 was that, if you run the trustymail command in the description of that pull request, the check for MX records raises a dns.exception.Timeout exception.

If you run it like this

$ trustymail --json --debug --timeout=30 --smtp-timeout=5 nara-at-work.gov

while connected to the NCATS VPN at least, you get a dns.resolver.NoNameservers exception. This leads to a different live result between the two cases because the two exceptions are handled differently.

I guess I need to dig in some more and figure out why the two different DNS hosts give different responses.