Calendar sharing is no longer supported in Outlook.com. It started as an upgrade and temporary break, but half a year later, after contacting Microsoft support, nothing has been resolved and the loss of calendar sharing seems to be permanent.
It started quietly without any warning from Microsoft’s side. All calendars in the Outlook Calendar app on Windows 10 Mobile stopped synchronizing with the Outlook.com service. The reason was a broken Microsoft account. The operating system said, You need to fix your Microsoft Account, but after doing that, nothing happened and everything was still desynchronized. A few days later, the issue was solved, but all contacts lost profile pictures.
I have been searching for what happened. HoTMaiL, later known as MSN Hotmail, then rebranded to Windows Live Hotmail, then to Windows Live Mail, and finally to Outlook.com, has been using the largest SQL Server instance in the world. Relational databases aren’t designed to be scalable. In May 2015, Microsoft announced it would move the service to an Office 365-based infrastructure.
The problems with shared calendar syncing were expected, but no one bothered to inform end users. The interruption was expected to happen only during the upgrade process. The sharing should be working again when all users were migrated. But it did not happen.
The desktop Outlook can communicate with the server using at least four protocols. The first is POP/SMTP, the second IMAP/SMTP, the third is Exchange ActiveSync, and the fourth is Microsoft Exchange. The first two options, POP, IMAP, and SMTP, are very well known and clear. I don’t know the technical difference between Exchange ActiveSync and Microsoft Exchange, but Exchange ActiveSync was used by Outlook.com before the upgrade and the endpoint was eas.outlook.com. Microsoft Exchange is used by Office 365 and thus by Outlook.com after the upgrade. Microsoft was sending emails to users connected by Exchange ActiveSync that they needed to reconnect to Microsoft Exchange. This is a trivial task and I did it as soon as I noticed my account had been upgraded.
After I reconnected to the new protocol, I wasn’t able to edit shared calendars. Even when I created a new calendar and shared it with other already upgraded users, it did not solve anything. The shared calendar was read-only for anyone but the owner in Outlook 2016 version 1701 build 7766.2060 for desktop PC. Editing the calendar was possible only via a web browser. The Outlook Calendar app for Windows 10 Mobile version 1607 build 10.0.14393.693 is unable to save events to the shared calendar. It pretends to work, but the event is never shown in the Outlook.com calendar.
I contacted Microsoft support to solve the problem or warn about an issue (case number 1367529784). At the start form, there is a statement: The more you write, the more we'll be able to help. That’s good, but the text area has a very low limit on text length, so I wasn’t able to precisely describe my problem. When I reached a very kind person from support, I had to explain everything again because he didn’t receive the description of the problem I wrote earlier. I spent one hour describing and demonstrating the dysfunctionality, and the result was simple. The official Microsoft solution is to downgrade back to eas.outlook.com and use Exchange ActiveSync again. When I explained that this protocol is going to be discontinued soon, I was reassured that this issue would be reported internally to the product group. A few months later, Exchange ActiveSync was really discontinued, but the shared calendar is still in read-only mode regardless of my efforts to make it work again.
I would expect that Microsoft would be clear about this issue and tell us when the migration will be finally done and what should still be working and what is not expected to work anymore. Instead, the Outlook team is focusing more on Mac and Google users.