Back in January 2013, there was talk that Microsoft was considering releasing a version of its Outlook mail client for Windows RT . It looks like talk is advancing into likelihood.

According to Windows SuperSite's Paul Thurrott, Outlook RT is being tested outside of Microsoft now, not just internally as it was a couple of months ago.

Thurrott also said he heard that Microsoft delayed releasing Outlook RT due to a firmware issue in ARM that was triggering crashes. I had heard scuttlebutt that excessive battery usage might be at fault, but nothing confirmable.

One of my sources said that Outlook RT has turned into a "top enterprise request," so the Office team had decided it should move forward and commercialize the port of Outlook to ARM. (I'm not so sure how many enterprises are really using Windows RT devices at this point, but I've heard Microsoft plans to increase its business-focused marketing push for Surface RTs and other Windows RT devices.)

This same source said all of the Office apps -- not just the core Word, Excel, PowerPoint and OneNote -- have been ported to ARM already. Once the initial work was done to shift to the ARM compiler, the task of porting the apps to ARM was less arduous, given the fact that huge chunks of each app is shared code. This shared code was developed by the Modern Office Experience (MOX) team and is known as MSO (Modern Shared Office?), my source said. MSO supposedly implements huge amounts of shared code for the office apps -- all the file access code, registry access, graphics, interfaces, etc. (Update: The MOX work has to do with Metro-Style Office, not ARM. At least right now, the Outlook RT app is believed to be a Desktop, not a Metro Style one. My mistake.)

"In short, these (Office) apps have been runable on ARM for a long time," my source claimed.

If it is released commercially, Outlook RT would run on Microsoft's ARM-based Surface RT devices, as well as any/all other ARM-based Windows RT tablets and PCs. It would likely be positioned as a complement, not a replacement to, the built-in Windows Mail client on Windows 8 and Windows RT PCs. Currently, Microsoft doesn't include Outlook as part of the Office Home & Student 2013 RT suite that it bundles with the Windows RT operating system. Only Word RT, Excel RT, PowerPoint RT and OneNote RT are included.

It's worth noting these four apps are Desktop apps, not "Metro-Style"/Windows Store apps, but Microsoft is working on Metro-Style complements to all four , which it is expected to roll out this fall as part of its Gemini effort.

The Office team is not commenting on Gemini or Outlook RT.

I recently heard from one of my own sources that Outlook RT might be released this fall, possibly in conjunction with the Windows 8.1 (Blue) and/or the Office "Gemini" updates. However, if Microsoft opted to deliver Outlook RT earlier -- maybe as part of an update to the Office 365 consumer and business subscription offerings -- it could even come to market sooner.

As Thurrott noted, the most likely way Outlook RT would probably be rolled out is as a paid subscription offering. And once this happens -- again, if/when it does -- the road for Microsoft to roll out some kind of an Office for iPad via a similar subscription model should be cleared (at least to some degree). Microsoft would still have to come to some kind of agreement with Apple over exactly how this would be sold if any code is to be made available via Apple's App Store.

Previous leaks about Office for iPad have pointed to Microsoft releasing only Word, Excel, PowerPoint and OneNote apps for iOS ; Outlook hasn't been mentioned (to date.) If Outlook is to be delivered for iPad someday, one would assume Microsoft wouldn't want to deliver it on the iPad before it delivers it on RT. That would be counter to Microsoft's usual position of "first and best on Windows." But it's a new Microsoft these days, so never say never....