Draft: Do not merge! Make validate_email function stricter.

The validate_function would pretty much accept anything that containes one or more @ chars somewhere after the first position. This change replaces the old validation with a regex. Though not perfect, it should probably work fine for the large majority of cases, while being stricter than the previous version.

While it would try to resolve the domain, which would catch any invalid domain names, it's ability to do so depends on which platform it's running on. This change makes the validation a wee bit stronger in the cases where it is not able to perform the dns lookup.

Also clarify the doc comment, that it does not actually try to resolve the email address, just the domain.