Email Address validation; added more comprehensive email address validation

Added a more comprehensive email address validation. This not only checks the email syntax, but that the domain exists and has mail records, and that the email server is valid and the mailbox exists at the mail server.

Note, the email address validation is not 100% accurate and may result in occasional false positive or false negative results.

This has been incorporated into the Client Contact Method drilldown form. There is a new "Check" button for Email addresses.