QUOTE from link above:
"Hotmail was still one of the most widely used services, with over 300 million active accounts," he wrote. "This made the magnitude of the process incredible, maybe even unprecedented. This meant communicating with hundreds of millions of people, upgrading all their mailboxes and making sure that every person's mail, calendar, contacts, folders, and personal preferences were preserved in the upgrade. Of course, this had to be done with a live site experience that was handling billions of transactions a day."
So, farewell Hotmail, which almost certainly had users active on it since its 1996 debut.
Microsoft just upgraded everyone who uses Hotmail to the new Outlook.com. This required migrating more than 150 petabytes (150 million gigabytes) of data.