Include MX record check

THis plugin only check if the domain name exists, not if it has an MX record. Without having an MX record it is impossible to receive emails.

Change link 97 in file /classes/class-email-dns-verification.php from:

return $response = ( checkdnsrr( $domain[1] ) ? true : false );

to

return $response = ( checkdnsrr( $domain[1], 'MX' ) ? true : false );