Keep your email lists up to date and remove email addresses which do not exist any longer.
You just have to define the path of your email lists. As a result you receive a list with guaranteed valid email addresses, a list containg probably valid email addresses and a list with invalid email addresses.
The EmailVerifier uses a proper proofing mechanism, because most of the mail servers have disabled the standard checking command. Further it exposes mail servers, which give the "OK" return value for every email address..