In previous builds of Windows 10, there was a known issue with the default Mail, Calendar and People apps which caused them to become corrupted and you had to use PowerShell to resolve the issue by removing the old app instances and re-installing them from the Windows 8.1 Store. My PC downloaded Build 10049 overnight and this build seems to have the same issue however the catch appears to be that if you follow the old instructions that it doesn’t work off the bat and you have to repeat the process as suggested on the thread http://blogs.windows.com/bloggingwindows/2015/03/30/windows-10-technical-preview-build-10049-now-available/.
First, open a PowerShell prompt with the Run As Administrator option. Once launched, enter the following code.
Get-AppxProvisionedPackage -Online | Where-Object {$_.PackageName -Like "*WindowsCommunicationApps*"} | Remove-AppxProvisionedPackage -Online
Once you have completed this, restart the PC. With the restart complete, open the Administrative PowerShell prompt and re-enter the same command again. I had to do this twice in the end so just hit the up arrow to re-use the command and hit enter to run it twice.
Once you have done this, open the Windows 8.1 Store using the green tiled Store app, not the Windows 10 Store with the grey tiled Store (Beta) app. In the store, search for Mail and install the Mail, Calendar and People app collection.
If the installation fails, try restarting and re-running the PowerShell command above as it will work eventually.
Once the apps are installed, looking at your Start Menu, you may see them appear corrupted still, showing the odd looking app names. If this is the case, unpin them from your Start Menu by right-clicking on the tiles and select the Unpin from Start option and then re-pin them to the Start Menu by right-clicking the odd looking app name from the All Apps list and select the Pin to Start option. Once you re-pin the apps, they should change to show the correct app name and launching the apps should now work.
Microsoft are reporting working on fixing the issue to prevent the corruption of these apps in future builds but for the time being, it looks like removing the apps just once that worked in previous builds isn’t enough.