Bad news guys.... Look at this:
http://10minutemail.com/10MinuteMail/index.html
With this tool anyone can bypass any email verification system (after or before registration, does not matter) and then discard the email. I've added @nwldx.com to the excluded emails, but... How long it would take them to get some other fake email provider? Also I recall that Yahoo has some similar service...
The only posible way for keeping valid user emails is to deal with bounces in a daily basis.