When the class says it is not possible to determine whether the address is valid, it does not mean the address is invalid.
If the class says an address is invalid, it may mean that the remote SMTP server would reject messages if they were sent from the IP address of the machine on which you are running the tests.
If you try from different machines, the response may be different.
Make sure you are not trying to validate addresses from a machine that has a blacklisted IP address.
Consider looking up your IP address in Senderbase for instance:
senderbase.org/