* Fix email with empty domain name labels passing validation
`EmailMxValidator` would allow empty labels because `Resolv::DNS` is
particularly lenient about them, but the email would be invalid and
unusable.
* Add tests
* Bypass MX validation for explicitly allowed domains
This spares some lookups and prevent issues in some edge cases with
local domains.
* Add tests
* Fix test
* Increase DNS timeout from 1 second to 5 seconds for MX check
1 seconds is rather short when using a recursive DNS resolver which
hasn't got a cached result already available. Use 5 seconds instead,
which is the timeout value we use for outgoing HTTP queries.
* Add more precise error messages for invalid e-mail addresses
