Microsoft: We’ve migrated 300M Hotmail users to

It has been a bit of a rocky road, but Microsoft(s msft) now says it has successfully migrated a whopping 300 million active Hotmail users to its newer email service and now claims 400 million users of that service. According to a blog post announcing the milestone on Thursday:

“Today, we’re excited to announce that we’ve completed upgrading all Hotmail customers to Coupled with the growing organic excitement for, this has pushed us to over 400 million active accounts, including 125 million that are accessing email, calendar and contacts on a mobile device using Exchange ActiveSync.”

And, the transition required the migration of 150 petabytes of data over in six weeks, no small feat. Given the comments on our earlier story about the transition (or “forced migration” as some put it), there are more than a few disgruntled Hotmail users out there, an issue Outlook Group Product Manager Dick Craddock addressed in the blog:

“Of course, whenever a widely-used consumer service makes any substantial change, there will always be some folks that don’t like it, and that shows up in the feedback, too. It’s gratifying in a sense because it means those customers loved the previous set of changes we made. With a communication service that is constantly evolving, we try to strike the right balance between bringing out major improvements and keeping true to what our customers love.”

Microsoft also more tightly integrated Outlook with its SkyDrive cloud storage and added support for SMTP send, which makes it easier to send a message from an alias. Microsoft, which dominated the work email landscape with its Exchange Server and Outlook combo, is facing really tough competition as Google Mail and Google Apps have taken hold. microsoft outlook This story was updated at 10:37 a.m. PDT to correct the number of users. The 400 million count used earlier represents more than 300 million active Hotmail accounts plus new customers.