The Outlook Mail and Outlook Calendar app store update released Tuesday (17.6417.42252) to Windows Insiders on Windows 10 Mobile build 10581 and above contained a severe crash on boot for a large number of users. We were quickly alerted to the issue from Insider feedback (thank you!) and we have pulled the build from the app store and will re-release our previous major update via the app store as soon as possible to unblock the affected users. When this occurs, you may notice many of our latest feature updates, like linked inboxes and month view in Calendar, will temporarily disappear. This is temporary and we will re-release to Insiders as soon as we have these issues fixed.

We certainly never want to introduce severe bugs like this, but we certainly appreciate having the Windows Insider community as a line of last defense to catch issues like these (usually much less severe bugs) before releasing world-wide to all users. 

As you have perhaps noticed in the past months, each of our major monthly updates we release first to Windows Insiders, sometimes with multiple bug-fixing refresh updates in fast succession, before we open up the release to all users. This has been a very helpful to us and we thank the Windows Insider Community.