Microsoft has completed the transition to its new Outlook.com email service. The successor (and replacement) to Hotmail, Outlook.com now has more than 400 million active subscribers. Microsoft opened the Outlook.com service in beta form last year, and finally took it out of preview mode in February. Since then, the company has been moving everyone’s accounts over to the new service. In all, the company moved more than 300 million accounts from the old Hotmail databases to the new Outlook service. Over a six week period, Microsoft moved more than 150 Petabytes of user data to the new service!

From now on, users will now log in to Outlook.com and interact with the new Modern UI-esque user interface. Users that were part of the company’s Hotmail service will get to keep their existing @hotmail.com accounts and no configuration setting changes will be necessary. New users will only get @outlook.com addresses, however. Any Hotmail Plus users will get to keep their paid status and enjoy a version of Outlook.com without any sidebar ads.

Now that the transition is complete, Microsoft is working on adding new features to Outlook.com. Right now, the company is working on introducing deeper integration with SkyDrive as well as tweaking the sending of email from alternate accounts. Both new features will be gradually rolled out to users over the next few weeks.

The SkyDrive integration will be bolstered by adding a new attachment option when sending an email that will allow users to attach files stored on SkyDrive. Outlook will then add a link to the email and automatically assign the correct permissions to allow the email recipient to download the file. If you attach a photo from SkyDrive, it will automatically create a thumbnail or gallery of photos within the email body.

The new SMTP send feature tweaks the way Outlook sends mail via an alternative email account (for example, if you added an old Gmail or Yahoo mail account to your Hotmail or Outlook.com email account) such that it no longer shows your Hotmail address “on behalf of” your alternative email. Once the new features is rolled out, email recipients will only see your alternative email address and your Hotmail/Outlook email will not be revealed.

If you are curious about the new Outlook.com interface, check out my Outlook.com preview article.