sent out emails for few weeks to them...to check the percentage of bounce back... percentage of email open and not open... then from there... you monitor and review your own whether want to do another exchange emails to that person again....

first you should sign up for an account here: Free Email Marketing Software & Services | ReachMail

Their servers are not the best for sending on, but the have a really good email filtering tool. The service is free so won't cost you a thing.

I use Reachmail my self with pretty good results. I would certainly recommend it and its free.

the only way to verify list is to send email , if the emails bounce back it means dead

You can have a try Email Worker , this software can check the email valid without sending real emails.