More then likely they're spammers. Spammers typically will never activate their accounts. Have you tested registering with a hotmail account to ensure those addresses are working?

There's nothing domain specific behind the registration emails, so no there's nothing to cause hotmail to reject the emails. Ultimately it is your mail server provided by your host which handles the sending.

It's possible your email could be classified as spam based on content. You can adjust the registration emails however from within CB configuration on the Registration tab (Pending and Welcome emails).