Scenario a) Microsoft wants to acquire Xobni, but Xobni does not want to be acquired at its present valuation (whatever Microsoft was offering). However Microsoft also has strong leverage over Xobni as it controls what components work with Outlook. So this scenario of an aggressive Microsoft being rebuffed by a confident startup does not sound plausible.

Scenario b) Microsoft is timing its acquisition. If Xobni is independent, it could possibly integrate its service with other webmail providers. These providers (like Google or AOL) would be very reluctant to let a Microsoft owned Xobni have access to their users. In this case, by delaying the acquisition Microsoft could be hoping to gain leverage over the other email companies once their users are hooked on Xobni. Something along these lines seems much more plausible to me.

A third scenario comes to mind: Microsoft realizes that its history of acquisitions has not been too great, and it thinks Xobni is too important to risk screwing up this way. This seems unlikely just based on everything I've read and heard about the company and its culture.