So today I auto-updated my desktop to the March release of Windows 10 Enterprise Technical Preview and sadly, even though it looks pretty, it isn't a hassle free update. 1, you need to re-install your anti-virus, 2, to get the build-in windows, mail and contacts to work again, you need to run the following code as an Administrator in Powershell.
Get-appxprovisionedpackage .online | where-object {$_.packagename .like .*windowscommunicationsapps*.} | remove-appxprovisionedpackage .online
You should get a simple return stating everything is ok. Path: Online: True and Restart needed: False. If you get something else, please doublecheck if you ran Powershell as Administrator.