Outlook is slow to open, especially switching folders in a 50 GB shared mailbox with 200,000 messages.

Outlook in cached mode on a large shared mailbox loads the whole OST locally. That's fast in principle, but at 50 GB you get slow indexing, heavy sync and a disk filling up. Disabling cached only for the shared mailbox is usually the fix.

Try this first

1. 1File, Account Settings, Account Settings, double-click your Exchange account, click 'More Settings', Advanced tab.
2. 2Make sure 'Download shared folders' is OFF. That loads the shared mailbox online (server-side) instead of as gigabytes of local OST.
3. 3Set the cache slider to 1 or 3 months. Caching the entire archive locally doesn't scale past 20 GB. For older mail Outlook uses online search.
4. 4Close Outlook, delete the existing OST in %LOCALAPPDATA%\Microsoft\Outlook (close Outlook first or the file is locked). On next start it rebuilds a smaller OST. One-time 30 to 60 minutes.
5. 5For structurally large mailboxes (>50 GB), discuss archiving with the owner: old mail into an Online Archive or splitting into a mailbox archive. That's not an IT fix, it's a process call.
6. 6Disable indexing for the OST if search isn't critical, or add the OST to the Search index if it's missing (Control Panel, Indexing Options, Advanced, File Types, OST on).

None of the above fits?

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