Exchange 2026 Shared Calendar With External Users Not Syncing Assessment Overview. Based on your description, you shared a calendar to external user (both of you are using office 365), the instant syncing is not enabled for different organization and you can check the detailed syncing information about it: Then make sure the following two items are not selected “download shared folders” & “turn on shared calendar improvements” save your settings and restart outlook.

On the calendar page, choose whether you want to let users share their calendars with people outside of your. According to your description, it seems that this issue is caused by the shared calendar didn’t sync into the user’s mailbox. The solution to this problem is to enable aclable object synchronization.