Microsoft's Hotmail, the free webmail service used by hundreds of millions of people around the world, was phased out Friday, as the US tech giant completed a rebranding to Outlook.com.

"We're excited to announce that we've completed upgrading all Hotmail customers to Outlook.com," Microsoft's Dick Craddock said in a blog post late Thursday.

"Coupled with the growing organic excitement for Outlook.com, this has pushed us to over 400 million active Outlook.com accounts."

The transition began in February, when Microsoft began a test of moving users to the new Outlook.com services.

"Hotmail was still one of the most widely used services, with over 300 million active accounts," Craddock said.

"This made the magnitude of the process incredible, maybe even unprecedented. This meant communicating with hundreds of millions of people, upgrading all their mailboxes -- equaling more than 150 million gigabytes of data -- and making sure that every person's mail, calendar, contacts, folders and personal preferences were preserved in the upgrade."

Outlook.com addresses will be better synched to other Microsoft services, such as its Bing search engine and Skydrive, the cloud storage system.