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. Microsoft 365 itself is usually not the root of the problem -  the mail apps and settings, sync issues, etc can be the root cause of the issues.   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 Apps

Microsoft Outlook (Windows and Mac), Outlook on the web, and the Outlook mobile app are the recommended apps. Keep current with Microsoft Office Updates. At this time (June 2024) Outlook New (Win) is not recommended – there are still too many bugs and missing features.

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 if you are managing another user’s 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

How many delegates should I have? 

• Outlook classic – 1 editing delegate, 4 reviewers. If you are giving only reviewer permissions, then those permissions can be set through the calendar and not through the delegate permission settings. Having multiple people with editing permissions and who receive notifications can lead to issues with troubleshooting issues and can lead to easier corruption of meetings
• It is recommended that 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 in general. Obviously, there will be times that someone else needs to accept/decline the meetings. It is important to try and limit multiple people doing it at once. All other computers/devices and people should ignore (e.g., do not process, do not delete) meeting requests for the mailbox if they receive them.
• For the new calendar sharing it is recommended to use the simplified permissions levels (can edit, can view titles and locations, etc.) rather than customizing permissions for delegates.  Custom permission levels are not supported in new model sharing and can lead to unexpected results. 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.

Delegates should be running in cached exchange mode for optimum performance.

Reduce the number of open calendars and remove any you don’t view on a regular basis. Each calendar added is an active connection and these should be limited in number.  One option is to use the Scheduling Assistant tab instead of opening each calendar (see below)

 

Handling Meeting Requests

Always Respond to Meeting Requests in the Inbox - Microsoft recommends 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 the 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 apps.

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 given the choice. Outlook does have enhancements where it may decide on the backend who to send the updates and not give you a choice. 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.

Changing the meeting organizer – Outlook doesn’t allow you to change the meeting organizer. The original meeting must be cancelled by the original organizer and re-scheduled by the new organizer.

Do not copy meetings. Outlook removes any links between a copied meeting and the original meeting. The later versions of Outlook (version 2311 and higher) no longer support copying meetings.

Meetings should NOT be made Private if reserving a resource – if a private meeting is scheduled and includes a resource, the resource delegate does not receive notification of the meeting, so they aren’t aware they need to accept/decline the meeting request.  This is working as designed (per Microsoft).  The resource delegate may not be able to view the meeting at all if set to Private. 

Private meetings – if you must schedule a private meeting, schedule two meetings.  The first invite the resource with general information on who is reserving.  The second meeting should be the attendees and details of the meeting that you mark as private.

Use the Scheduling Assistant tab while creating a meeting.  When trying to find an available time for a meeting, instead of opening each person and possible resource that needs to be invited to a meeting, use the Scheduling Assistant tab. This allows you to enter the attendees and possible resources so you can see the availability of all of them at once. 

Recurring Meetings

Always Schedule end dates on recurring meetings - It is recommended that you set an end date (no more than 6 months) when you schedule a recurring meeting. If you have a weekly recurring meeting and it requires updates to the agenda or meeting, we recommend making even shorter recurring meetings (e.g., 4 weeks at a time) to reduce corruption.

To change an entire series of meetings, you can: 

1. Cancel the original meeting series and create a new one.
2. If you need to keep meeting history, edit the series and change the end date of the series to the current date. This will delete all future occurrences but keep the past meetings for historical purposes.
3. To change one instance, cancel just that meeting and create a new one to replace it.

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 1-3 months instead of 6 months to 1 year or more. to reduce corruption.

Not all calendar items have an equal effect on performance. One-time meetings have a relatively small effect, and long-lived, recurring meetings have a greater effect. A change that's made in an instance of a recurring meeting creates exceptions that are stored in the recurring meeting series. Every change to the meeting attributes, such as the subject, body, location, or time, creates a corresponding exception. Eventually, these exceptions inflate the size of the recurring meeting and impose a greater performance cost. You can mitigate this higher cost by using Appointment Recurrence settings to set an end time or maximum recurrence count. Then, you can create a new recurring meeting instead of extending the existing recurrence – per Microsoft article

Recommended maximum of 25 occurrences for recurring meetings but if frequently edited or changed, should be less. You should always use an end date on recurring meetings.

For recurring meetings, attachments are not recommended. Microsoft recommends sending the attachment in an email or link to OneDrive instead of in the meeting itself.  Each exception to a recurring meeting contains its own copy of the attachments. If you make changes to an attachment in one of the meeting exceptions, it only changes that instance, it doesn’t get changed in the other instances.

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.

Meetings should NOT be made Private if reserving a resource – if a private meeting is scheduled and includes a resource, the resource delegate does not receive notification of the meeting, so they aren’t aware they need to accept/decline the meeting request.  This is working as designed (per Microsoft).  The resource delegate may not be able to view the meeting at all if set to Private. 

Private meetings – if you must schedule a private meeting, schedule two meetings.  The first invite the resource with general information on who is reserving.  The second meeting should be the attendees and details of the meeting that you mark as private.

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 then delete the attachment from the meeting.