No Email Validation When Incorrect Email is Entered while Testing Configuration

The provided patch file "fix_mail_validation-3403990-2.patch" adds the email validation when the Recipient field is not empty and will throw an error message for invalid field value.

Thanks for the bug report. The test email field is an HTML field of type "email" which does its own validation. It does allow some unconventional email addresses, such as me@localhost, which can be a valid testing email address. It's a bit loose in some ways and a bit tight in others but I don't think adding extra validation here is necessary.

I'd also prefer not to add a regex to this module, aside from what the PHPMailer library provides.

For more information: https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address