Our dean's assistant manages the dean's Microsoft 365 calendar through delegated access in Outlook Classic. The calendar and mailbox data total more than 7 GB, and Outlook frequently becomes unstable or loses functionality—for example, she currently cannot add locations to appointments. Removing and re-adding the calendar takes so long that it is rarely practical. We have tried different cached Exchange settings and limiting the visible date range, but she insists on having three years of appointments available. Microsoft support has said the calendar is simply too large and complex to support reliably. New Outlook works better, but she dislikes its workflow. Has anyone successfully used another interface, configuration, archiving strategy, or third-party tool for managing a calendar of this size?
4 Answers
Consider enabling an online archive and using retention or archiving policies for older mailbox content and appointments. That can reduce the active mailbox workload, although it may not completely solve a calendar that is itself unusually large or contains a huge number of recurring and historical items. Test it carefully before assuming it will fix the calendar view.
Since this is Exchange Online, compare the same delegated calendar in Outlook on the web and New Outlook before buying a third-party product. If both modern clients work normally, that points strongly to a Classic Outlook and MAPI limitation rather than a permissions problem. You may need to present the business owner with a choice between the supported modern interface and a reduced date range.
At some point you have to set a firm limit on either the date range or the amount of data being loaded. Keeping three years of a heavily used delegated calendar available in the desktop client may simply be beyond what Outlook Classic handles reliably. A web-based interface could be a reasonable compromise if the workflow is acceptable.
The most practical fix is probably New Outlook. Classic Outlook relies heavily on MAPI, which tends to perform poorly with very large mailboxes and complex delegated calendars. New Outlook uses Microsoft Graph and may provide a much smoother experience. The main thing to verify is whether the assistant depends on any Classic-only features.
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