If you've noticed missing calendar appointments, e.g., lost meetings, delegate issues, or meeting updates from someone other than the original organizer, you're not alone. This issue has been reported in organizations with Microsoft 365, Microsoft Exchange, Outlook (2010, 2013 or 2016), Apple iPhones/iPads, Active Sync devices, Macs running Outlook for Mac, etc. For more information see: Microsoft Calendar Appointment Corruption Issues (August/2018 Update) and Exchange Outlook Calendaring Problems.
Microsoft 365 itself is usually not the root of the problem - the mail clients (ActiveSync, iOS devices, iCal) and their connectivity protocol tend to be the culprits. Another cause of the issue is sync timing. For instance, if someone accepts a meeting at 9am on an iPhone and a delegate declines it at 9:10am on a PC, but the iPhone is out of signal range and doesn't sync the meeting until say 9:30am., what happens to the meeting? Many people assume that the Office 365 calendar system tracks a meeting as a single copy and that any edits to a meeting will automatically update and appear for all attendees. Instead, in order to allow Microsoft 365 meetings to work with other calendaring systems and external users, all calendaring data requires the sending and receiving of email messages to all attendees. As each attendee receives an email message with calendar appointment information, their email and calendaring client reads the message and updates their individual copy of the meeting on their personal calendar.
Some best practices are listed below:
Use Preferred Clients
Microsoft Outlook, Outlook on the web, and the Outlook mobile app are the recommended clients. Keep current with Microsoft Office Updates - There are known issues that are fixed with each service pack or update. Make sure your mobile device has the latest OS/iOS version.
Use the same version of Outlook - Mailbox owners and any delegates should be using the same version of Outlook with the latest service pack and updates on all computers that are used for calendaring. If you are in a mixed environment of Windows, Mac or mobile devices, each platform should use the same version and each device should have the latest service pack and updates.
If you must use a mobile device to manage your calendar, use the Outlook app to accept/decline meeting requests. It is recommended that mobile devices only be used for viewing your calendar. Mobile devices tend to cause the most issues with appointments becoming corrupt, missing, or out of sync. Scheduling an appointment just for yourself on your mobile device is ok.
Delegates
Only ONE person should process meeting invites - Make sure that only one user for each mailbox receives and processes meeting requests. If you have more than one delegate, having only one receive the meeting requests is preferred. If more than one receives the requests, the delegates and owner must decide who will be processing all the meeting requests. All other computers/devices and people should ignore (i.e. do not process, do not delete) meeting requests for the mailbox if they receive them.
If more than one user must have access to your mailbox, carefully consider whether these users have to be delegates or if you can assign Reviewer/Editor permissions instead. See Permission Settings For Your Microsoft 365 Calendar for more information.
Handling Meeting Requests
Always Respond to Meeting Requests in the Inbox - We recommend that you always accept or decline a meeting request from the Inbox. If you accept or decline a meeting by using the meeting item in the Calendar in Outlook, the meeting request remains in the Inbox. It is important that you do not delete a meeting request from the Inbox until you are sure that the meeting has been processed. The following text appears in the InfoBar of the meeting request when the meeting request has been processed: Accepted by username on date, time
Don't move meeting requests - Don't move a meeting request from your Inbox to a different folder before you accept or decline the request or before the meeting appears in your calendar. Soon after a meeting request arrives in your Inbox, a piece of Outlook code — nicknamed the "sniffer" — automatically adds the meeting to your calendar and marks it as tentative. This is a fail-safe to keep you from missing the meeting in case you don't see the request in your Inbox. However, the sniffer doesn't reply to the meeting organizer. You still need to do that by accepting, accepting as tentative, or declining the request. If you or a rule that you create moves an incoming meeting request from your Inbox before the sniffer can process the request, the meeting never appears in your calendar, and you might miss the meeting.
Don't delete a meeting request on one computer after you accept the same meeting request on another computer - If you are using two computers that connect to the same mailbox (e.g., a desktop computer that is using Online mode and a laptop computer that is using Cached Exchange Mode). The meeting request that you accepted on the Desktop computer is immediately processed. The meeting request that you deleted on the laptop computer is synchronized later. After synchronization, the meeting on the desktop computer is also deleted.
If you receive a meeting cancelation, click Remove from Calendar to remove the meeting from your calendar - Deleting the cancelation from your Inbox won't remove the meeting from your calendar when using some clients.
Don't auto-accept meeting requests - If you have granted one or more persons delegate access to your calendar or if you have delegate access to someone else's calendar, turn off automatic acceptance of meeting requests. By turning off automatic acceptance you avoid problems with delegate workflow by allowing you and/or your delegate the opportunity to review all meeting requests, respond appropriately, keep track of meetings and any changes, as well as minimize schedule conflicts.
Do not forward meeting requests - the meeting attendee list becomes out of date and tracking does not work properly for the forwarded users. Any updates to the meeting are only sent to the original attendees, not the forwarded users.
Creating and Editing Meetings
If you cancel or delete a meeting, ALWAYS Send Update to EVERYONE - If you cancel or delete a meeting, but you do not send the update, the meeting is only removed from your calendar. If you are the organizer, no one is notified that you are canceling the meeting. If you are an attendee, no one is notified that you are not attending.
If you didn't organize the meeting, don't modify your calendar item - We recommend that you do not put personal notes in the body of a meeting item in the calendar. If you are an attendee of this meeting, your notes will be lost if a meeting update is received. If you are the organizer, your personal notes will be sent to everyone on the attendee list.
Do not invite a LISTSERV list to the meeting. You may not always know who is a subscriber on the list and tracking information of responses isn't reliable. The LISTSERV members also all end up as Optional Attendees.
Recurring Meetings
Schedule end dates on recurring meetings - Microsoft recommends that you set an end date (no more than 6 months) when you schedule a recurring meeting. When you add a definite end date, you may prevent issues that may occur if you have to update the meeting several times. If you schedule an end date on meetings, you can create a new meeting if you realize that the meeting has to be frequently modified.
To change an entire series of meetings, cancel the original meeting and create a new one. To change one instance, cancel just that meeting and create a new one to replace it. Always put an end date on a recurring meeting.
A "corrupt" meeting will remain that way until you delete it. If it is a recurring appointment, delete all occurrences and reschedule it.
Avoid frequent changes - Recurring meetings can become lost or duplicated if they are modified too many times. If you have a series that requires many changes, like multiple location changes, cancel the series for everyone and create individual meetings instead or shorten the recurrence period to 3-6 months instead of 1 year or more. .
Resources
When reserving a resource, invite the resource as a Resource attendee. Do not schedule the meeting directly on the resource calendar. Scheduling directly on the resource calendar bypasses the meeting conflicts check and you may end up double-booking a room. Invite the room as a resource instead.
Recurring Meetings -- when scheduling a recurring meeting with a resource, be sure to open the acceptance message. If there is even one conflict with the dates of the recurring meeting, the resource will decline ALL meetings. Pay close attention to the meeting responses.
Miscellaneous
Archive or delete older meetings and appointments. For best performance, Microsoft recommends having less than 5000 items on a Calendar. This is especially helpful for others viewing your calendar.
Don't send attachments with meeting invites. Store and share attachments via OneDrive or some other network or cloud space that attendees can access. This helps keep the size of people's calendars smaller and allows for attachment updates without resending meeting requests. If you receive an invite with an attachment, save it to your network space then delete the attachments from the meeting.
A corrupt meetings will remain that way until you delete it. If the corrupt meeting is a recurring meeting, delete the entire series and reschedule the series.