This hack does a live test of a user's mailbox using the MX record for the supplied domain (the part after the @). If there is a network issue or all the hosts are down or don't respond, it will return "false", meaning that the supplied email address won't be able to be checked until the connection works.

If this will be a problem, this hack can be enabled/disabled via the admin control panel. The default setting is "off" after installation.