Microsoft has completed the move from Hotmail to the new Outlook.com, which now boasts more than 400 million accounts.
The new Outlook email client has several different features from Hotmail, such as two-factor authentication, an updated calendar and app and integration with cloud service Skydrive and Skype.
Most Hotmail users will not notice much difference. They can continue to use those accounts as long as they choose and can claim an Outlook email address whenever they like.
Writing in the company blog , Dick Craddock said that Hotmail had more than 300 million active accounts.
Upgrading had 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. All this was done live and in only six weeks.
Hotmail was one of the first web-based email services. Founded by Sabeer Bhatia and Jack Smith it was launched on July 4 1996 as "HoTMaiL". Microsoft bought the web email service in 1997 for an estimated $400 million, and it was rebranded as "MSN Hotmail".
Outlook.com was launched in February 2013. It's based around Microsoft's Metro design language, and closely mimics the interface of Microsoft Outlook.