I don't know the answer for that, only wondering: what can you do with them?
They are dead, most probably users created by the spambots.
The email addresses are fictional or someone else's addresses.
Well, at first I also had the similar thought, but then few things made me rethink:
The email addresses are very well-formed (usually bot-generated emails use some random or typical phrases)
Most of the email address are of Hotmail domain, and I know Hotmail doesn't allow any email from my server unless you manually put my domain in your safe sender list
I have frequent posts from unconfirmed users (I have guest post enabled), which means either they are lazy or, most likely, they didn't see the email
I have few bot-proof custom fields like Full Name (with a notice, it must be different from the username), Country, Capital etc. I have noticed that usually bots put:
Full Name = user name
Country = Some well known phrase e.g. Chicago Bulls, LA Lakers etc.
Capital: Even if they put some valid country name, they almost certainly put some random capital name, like Bora Bora with Switzerland
That's why I want to send an email from my gmail box to all the users of past few of months before I delete'em all!