Firstly, to appease the SEC, I work for Microsoft and I'm actually working on this feature.
Our consumer offering Outlook.com (hotmail.com) has had this for years, so this project is specifically about adding the functionality for our Enterprise Exchange Online service. That said, it could have been added a while ago. Historically, one of the reasons we were dissuaded from adding support was that our customers have been able to create mailboxes in Exchange using the plus character and that threatened compatibility issues. We are making it an opt-in feature knowing that some customers neither want it nor can use it together with existing mailboxes with plus characters.
Other priorities also won over as this was not seen as a critical business user need. We have, however, been taking feedback from our Uservoice site (office365.uservoice.com) and this feature ask grew and grew. The feature is already complete and we're beta testing it at the moment. Please let me know if you have any questions.
On a personal note, I'm really proud that a feature I fought to get developed has made it on to Slashdot, where I have been getting my tech news for over a decade.
