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Microsoft Outlook Calendar Corruption, Lost Meetings, Duplicate Appointments - April/2015 Update

Over the past 5-yrs, organizations have complained about Microsoft Outlook calendaring problems where users describe issues of calendar appointments not showing up, meeting appointments disappearing, calendar delegate issues occurring, just overall "odd" behavior of calendars.  It usually happens in mixed environments where some users are Apple Mac users, and some users (frequently the exec admin / delegate) running Windows, and typically active use of iPhones, iPads, Android, or other mobile devices and tablets.  And over the past 5-yrs, I have actively blogged about the problem and the solution to FIX the calendaring inconsistencies.

This is a April/2015 update that I'm glad to post as there is FINALLY a great solution to the problem.

More specific to the noticed challenges:

  • Lost Appointments: Appointments sent to someone, the person accepted the appointment, but now the appointment isn't in their calendar anymore
  • Duplicate Appointments:  Having the same appointment show up multiple times in the calendar
  • Appointment Corruption:  Appointments (typically recurring appointments) that cannot be opened as an error notes "Cannot read one instance of this recurring appointment.  Close any open appointments and try again, or recreate the appointment"
  • Odd Delegate Issues:  Where a Boss/Delegate relationship is challenged by the delegate not getting appointments consistently, or approvals not showing up as approved

I originally blogged about the problem and the (then) solution back in 2010.  Here's a link to the original article:

Since writing that article back in 2010, I’ve since written several other “how to fixes” to the various problems including as well as

What is typically common with disappearing and corrupt appointments that happens between Exec and Exec Admin occurs when an appointment is scheduled on a Windows PC (or Mac), accepted on a Mac (or Windows PC or iPad/Android tablet/device), and then slight modification to the appointment from a different make/model of device occurs.  So an Exec using a Mac, with an Exec Admin using a PC will almost certainly corrupt the calendar appointment as they accept, change, modify appointments in the shared delegated calendar.  Or an appointment created on a Windows PC, accepted on an iPad, while either the Exec or Exec Admin uses a Mac will also surely corrupt the calendar appointment.

The above articles go through the technical details of the issue, that I clearly reference Apple Tech notes and Microsoft Tech notes of the root problems, however the challenges over the past several years is as soon as Apple fixes a bug on their end, Microsoft comes out with a patch or update that injects a new problem, and then Apple comes out with a new version of their email and calendar client (for iPads or Macs) that then causes corruption that Microsoft patches and fixes.  It was an ONGOING back/forth problem that was very difficult to keep on top of.

To summarize the technical issues that create the problem, when say for example a calendar appointment is created on a PC, that appointment has a unique message ID that links together the calendar appointment and any recurring instances and attachments to the appointment.  However various versions of the free calendar app that come with the iPad, iPhone, even the Calendar app native to Apple Macs (and even Microsoft's own Entourage and Office 2011 for mac) had varying behavior issues that do not always adhere to the associated appointment message identifier that Exchange (and Office 365) uses.  So in varying instances when the appointment is accepted and changed on an Apple device, the appointment ID is changed, the attachment is detached from the appointment and given a unique message ID, and recurring appointments are split up and saved as separate individual calendar appointments.  Effectively, the calendar appointment on the Mac or iPad is now disassociated with the original Windows-based calendar message ID.  When the appointment is changed, updated, modified, different behaviors kick in that delete the appointment on one type of device (but not others), or an appointment deletion on the PC side doesn't delete the new appointment ID that was created on the Mac, etc.  This is the root cause of the problem that I go in to more detail in the other blog posts and briefly summarized here in this update.

So how does one SOLVE this calendar corruption problem today???

The solution requires 3 fixes:

  • an updated and reliable backend environment (ie: the latest Exchange 2013 or Office 365 in the cloud)
  • use of the latest Microsoft provided client software (ie: Outlook 2013, Outlook for the Mac (2013 or 2016), iPad/iPhone/Android Microsoft provided Outlook client)
  • good user behavior

Many orgs have begun migrating off Exchange on-premise to Office 365 in the cloud that has solved a big portion of the problem.  With Microsoft's updated backend environment, a third of the problem has been addressed.  But that doesn't solve the entire problem.  As noted, organizations need to get to a consistent client software as well.  If users continue to use the built-in Apple clients against Office 365, they'll still get quirky issues with delegates because the free Apple clients (for Mac and iOS) have varying quirks to them that Apple fixes, then Microsoft fixes to work around the Apple fix, that Apple then works around to fix the fix that Microsoft injected, etc…

Here’s the best solution:

  • Update your client software to use ONLY the latest Microsoft versions of software.  That’ll include Office 2013 for Windows (latest patched and updated release); Office for Mac (latest March/2015 release or more current (I’d even use the beta version of the Office 2016 for Mac that came out last month at the time of this writing, while “beta”, it actually keeps the calendars solid (; and for iPads/iPhones also run exclusively the Microsoft (specifically the OUTLOOK for iPad that just came out a couple weeks ago    Using any Apple variant (the ones included free with the Mac, iPhone, iPad) or older Microsoft releases (ie: Outlook 2011 for Mac, OWA App for iPad) should no longer be used
  • After you get to the latest client software, THEN migrate the mailbox to Office 365.  Office 365 has proven to keep mixed calendars stable across platforms.  You can use Exchange 2013 with the latest updates, however Office 365 is more up to date and keeps enterprises cleaner for cross-platform calendars
  • When you migrate calendars, the suggestion is migrate all previous calendar appointments, but for anything today and in the future (including recurring appointments) to NOT migrate ANY of those.  To jot down (manually) all current and future appointments and resend them (with and from the most current version of the Microsoft clients when on Office 365).  This sounds painful, but usually it’s a few dozen appointments and can be done in a couple hours or less.  The whole goal is to NOT migrate over corruptions.  If you migrate over current and future appointments that were corrupted in Exchange 2007, Exchange 2010, Exchange 2013 or on the old Apple/Microsoft calendar clients, all you are doing is polluting the new environment.  Take the step to selectively NOT migrate over current and future appointments.  Get all users completely up to date on the client software, then send brand new appointments out.  This will maintain integrity of the calendars!

(note: before you do the migration to Office 365 and resend all new appointments, make 100% certain that ALL systems are absolutely running the latest version of clients)

We’ve seen orgs where some exec or some admin has an old iPad at home they “occasionally” use that takes a nice clean environment and corrupts calendars frustrating everyone until we uncover from the logs that the mysterious (old) tablet exists and is screwing things up.  The way to check is run through the Exchange logs, delete all pairings of devices to mailboxes that are unknown, and only allow known updated devices to connect to Office 365 thereafter (this is best done with a mobile device management policy solution (like Microsoft Intune, Airwatch, etc))

And lastly, the Exec and the Exec Admin also play a role in the ongoing success of calendar integrity.  As noted, the Exec/Exec Admins need to make sure they do NOT use any old device that is using any of the older/free versions of the email/calendaring app on their devices.  They CANNOT cheat and "occassionally" sneak in use of an un-updated system.  They HAVE to make sure they are using the most current, up to date, MICROSOFT client software that is available on each device.

And the Exec / Exec Admin have to treat calendars like any other shared document.  You would never think about taking an Excel spreadsheet, having 2-3 people using the spreadsheet at the exact same time, and wonder why cells and content are getting overwritten and corrupt.  Same thing with calendar appointments.  If the Exec and the Exec Admin inconsistently accept and modify appointments, AND potentially make changes at the same time, one (or the other) of the changes will take effect (but not both).  And unlike a shared document where you'll only get 1 (or the other) document, for calendar appointments, a Delete One recurring appointment by 1 user, and a Modify All recurring appointment by another user done at the same time WILL create very odd and interesting results regardless of how clean the backend and client software versions are.  So the USERS need to do their part.  Usually our suggestion is that only ONE of the two (Exec / Exec Admin) accepts or rejects all appointments, or modifies appointments, and that the other ONLY interacts with calendar appointments as read-only instances.  By truly delegating ALL functions to just 1 of the pair will ensure the "users" don't inject user-based corruption.

Over the 5-years that I've been blogging on fixes to this problem, the great thing is there is FINALLY a stable solution; utilizing Office 365 on the backend, and using actual Microsoft provided Outlook clients for various platforms on the frontend.  These, along with some good best practices of minimizing the bouncing around of execs and exec admins modifying / changing appointments at the same time (that also cause corruption), an organization can eliminate calendar corruption issues once and for all!

Hopefully this will be one of the last articles I'll be publishing on this topic, and that FINALLY we have a solid and consistent fix and solution to the problem.

If you have problems, input, comments, send me a message off this blog post, the messages end up in my mailbox, and I'll look to respond and lend a hand.  Hope you find this information helpful!


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