Lots of users registering but not activating acco.

We are a little worried because we've launched a web of a contest in which a lot of people is registering (we're using Community Builder), but a remarkable percentage of them are not validating their user account. We've checked it's mainly happening with people who have an email account in hotmail. With such a big percentage of created but unused user accounts the only thing we can think of is that many activation mails are not arriving.

Is there anything to do on this matter? Maybe hotmail rejects mails if their topic is written in a certain way, or the body...

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).