Microsoft has wrapped up its massive Hotmail user migration programme.

The company said that it has now transferred all of the user accounts from Hotmail to its Outlook.com service, ending a data transfer campaign which shifted some 150 petabytes of user data.

“When Outlook.com came out of preview in February, it already had more than 60 million active accounts,” wrote Outlook.com group program manager Dick Craddock

“However, Hotmail was still one of the most widely used services, with over 300 million active accounts. This made the magnitude of the process incredible, maybe even unprecedented.”

With the migration complete, Microsoft now estimates that Outlook.com boasts more than 400 million active users. The milestone comes some two months after the company reported that the service was over 60 million users.

Additionally, Microsoft reported that some 125 million of Outlook.com users also use the Exchange ActiveSync service.

To mark the occasion, Microsoft is posting an update for the Outlook.com service. The features will include a new interface option, the ability to send messages from other accounts without an ‘on behalf of’ notification and deeper integration with SkyDrive.

Craddock said that with the new SkyDrive integration, users will be able to more efficiently select and attach files and images from SkyDrve directly into messages.

“When you’re sending an email message, you can select files from your SkyDrive and we’ll automatically turn those into the right thumbnails with links that have the right permissions tied to people that receive the email,” he explained,

“When you insert pictures from SkyDrive, you automatically get a beautiful photo mail. And it’s easy to edit the message, and add or remove files and pictures right from the new message compose experience.”