User platforms are now fluid, so apps must be too
Yes, apps and services need to adjust to the devices they're running on, dealing with the real differences among them. But at their core the logic and capabilities should be the same. That doesn't mean the apps should be identical, but the core capabilities are the same, and the differences (both what is removed and what is added) make sense for the device's special context.
A cloud service, API, microservice, data center server, certificate, or data source shouldn't care much about what device is consuming it. Ideally, it would not care at all, and instead let the local app handle any local transformations or restrictions required.
Still, this is clearly a hard concept for many to grasp, both for vendors and within IT. It's not simply an issue with Outlook.
Take mobile banking, which is increasingly the default approach to banking. If you use a tablet, you know that many banks' apps are inferior to their websites -- yet the tablet can run both versions.
There's simply no reason for the native app to be less capable than the website -- in fact, it should be more capable. Even the smartphone version should not feel crippled; UI changes can address most of the actual platform differences due to the smaller screen and virtual keyboard, as opposed to stripping out features. The mobile app should do more than the desktop or Web app when that makes sense for the device's capabilities, from using fingerprint readers to location data. Some apps do, but many do not.
Think about how you use services like online banking: You likely gravitate to the most capable instance you can get for the device at hand, with the tablet usually being where that choice is clearest. Ditto for any app, from Google Apps to Outlook Web Access, from Slack to a game.
Why would you not expect your users to do the same?
A common objection is that supporting all those platforms is too hard. That was a major argument for the notion of making all apps into HTML5-based Web services that would allow a nearly one-size-fits-all approach.
But the truth is that browser differences are worse than platform differences in many cases, and browsers are much less capable than native apps. There are four platforms to develop for, all of which have facilities to deal with size and other form factor differences: Windows, iOS, Android, and OS X. Cross-platform IDEs are no cure-all, but they can help.
In a sense, it doesn't matter if it's hard to develop across platforms, because that's the world we and our users and customers live in.
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