Windows Insiders with Windows 10 Mobile can download a new version of the Mail and Calendar app. It brings back the linked inbox feature that was added recently, and then removed, from the public version of the app.
You may remember that the feature was taken away a couple of weeks ago as a temporary solution for the Mail and Calendar app crashing for some users. It looks like Microsoft is testing a new version of the app with Windows Insiders that hopefully keeps the linked inbox feature without all that crashing.
Officially, the version number for the app has changed from 17.6419.42012 to 17.6430.42252. The linked inbox feature allows users to link all of their email accounts into a single, unified inbox in the Mail and Calendar app.
Reminder: If you are using a Lumia 950 or Lumia 950 XL and are not on the Insider program you will not see the update! (To further clarify, both of those phones currently are not compatible with the Insider program either). The update is also only for mobile.
Thanks to everyone for the tip