Subscriber email addresses are verified when they are imported, and the top level domain (TLD) of the address is checked against a reference list to ensure that it is valid.
New TLDs come online frequently, and the reference list must be kept up to date in order to accurately validate subscriber email addresses which use them.
phpList has a built-in TLD reference list and automatic update mechanism to handle this processes conveniently. This feature was launched in phpList 3.0.9 (see the release notes for details).
By default the reference list of valid TLDs is udpated from phplist.com. You can change this to another reference list if you prefer by setting TLD_AUTH_LIST. This list is updated frequently. To check that the fetched list is accurate, a hash of the list is also fetched. You can change which hash is used by setting TLD_AUTH_MD5.
The frequency with which the list is fetched and updated can be changed by setting TLD_REFETCH_TIMEOUT.