code: public static boolean isValidWithMXCheck (@Nullable final String sEmail)
    {
      // First check without MX
      if (!EmailAddressHelper.isValid (sEmail))
        return false;
  
      final String sUnifiedEmail = EmailAddressHelper.getUnifiedEmailAddress (sEmail);
  
      // MX record checking
      final int i = sUnifiedEmail.indexOf ('@');
      final String sHostName = sUnifiedEmail.substring (i + 1);
      return _hasMXRecord (sHostName);
    }
docs: Checks if a value is a valid e-mail address according to a complex regular
  expression. Additionally an MX record lookup is performed to see whether
  this host provides SMTP services.
  
  @param sEmail
  The value validation is being performed on. A <code>null</code>
  value is considered invalid.
  @return <code>true</code> if the email address is valid, <code>false</code>
  otherwise.