The example given relies on a specific mail server behavior, that unfortunately, isn't the same for every mail server.
Essentially, what it's doing is connecting to the mail server of the given domain, initializing a message to that e-mail address, and hopefully the mail server will complain if it's not a valid e-mail address.
Otherwise, it's assumed as a good e-mail address.
I would assume that AOL's mail servers are behaving slightly different than the expected behavior, and thus the php script is return either false positives or false negatives (depending).
Hope this helps.
