---- dataentry configoptions ----
type             : variable
name             : $check_for_host
purpose          : Verify that the HOST part of an email address is a deliverable domain
allowed          : true false 0 1
default          : 0
status_s         : 
category         : subscriber management
----


This will add a little delay in the subscription process, as the domain needs to be looked up using the DNS system. You may also need to verify that resolving DNS works on your phpList server.