As above, there is no truly 'good' way to do this which isn't likely to cause more problems than it solves. You would need to get knee deep in the nitty-gritty of SMTP, and make a call on 'philosophical' questions such as 'does a mailbox which returns a soft bounce message count as existing?'.
Which is why the standard 'we will send you an email, click a link in it to validate' approach is normally used.
