Microsoft's Service Agreement for Outlook.com directs each user to sign in every 270 days (or earlier), unless they wish to have their account disabled, for good. Sadly, this little clause hasn't been well publicized, so many people were a little shocked when they find out that their year old Outlook.com account had been disabled. Inactive user account names are then made available to all new sign ups 360 days after they have been deleted/deactivated.
Another major email service provider was under fire for having done a similar thing in the past (psst, it was Yahoo!). Anyhow, we recommend all our readers to log-in to their old email accounts on Microsoft's Outlook.com at least once every 270 days, lest they want to find them deactivated permanently.