Most people come to us because their email service provider (ESP) has warned them about hard bounces. These are a real problem for your ESP and can get their servers blocked by some of the major ISPs such as Yahoo.  If your list contains more than a handful of invalid emails your ESP will ask you to verify the list before uploading into your account, or if not they may suspend your account.  If you run your own mail servers you will end up getting your IP addresses blacklisted by the major ISPs if you send more than a tiny percentage of invalid email addresses.