Relax email validation rules even more.

While filter_var() allows a number of email addresses that Validation::email() does not, it misses out of email address that contain IDN host names, and unicode mailboxes. Both of these are generally deliverable, and should be permitted. filter_var() also fails on local mailboxes like `root@localhost` which is useful in the context of cron jobs.

Fixes #3742
Fixed.