ValidEmail needs better diagnosis messages

[jace]

In the ValidEmail validator, we return the same error message regardless of what the diagnosis is. There are two situations where this is a problem:

1. Temporary lookup failure. This is confusing to users, who may try again and find that it works the second time. They should be explicitly asked to do so.

2. Missing MX record. This is very confusing to users, since they can receive email without MX and don't understand what is wrong. ValidEmail should explicitly point out that MX is missing.

Further, failure on missing MX should be optional and accepted as an init parameter.