Demanding valid email address activation.

How can I demand a valid email address activation when users are registering for a new account?
Like to send them those kind of emails where they have to go to a link to prove they own the email address.

Thank you for your time.

I would be interested in the answer as well. We had someone register using another member's email adress almost exactly. We haven't activated the account.

If you do not change any settings then it will automatically send them an email.

Mine is not sending any activation email. Can you tell me how to activate it?