An MX Lookup (not available for Windows servers) is used to evaluate the e-mail address that the end-user provides. For example, if they enter email@address.com, ListMessenger will check to ensure that address.com has a valid mail exchanger record in their DNS. This is to ensure that the domain that the end-user entered is a) valid b) is able to accept e-mail. This function can possibly slow down the script by a split second or so, but also helps to prevent false e-mail addresses in your list.