code: public static boolean isValid (@Nullable final String sEmail)
    {
      return s_aPerformMXRecordCheck.get () ? isValidWithMXCheck (sEmail) : EmailAddressHelper.isValid (sEmail);
    }
docs: Checks if a value is a valid e-mail address. Depending on the global value
  for the MX record check the check is performed incl. the MX record check or
  without.
  
  @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.
  @see #isPerformMXRecordCheck()
  @see #setPerformMXRecordCheck(boolean)