Reducing Outlook cache size for shared mailboxes

Classic Outlook keeps a local copy of email on each person's computer, called the cache or OST file. When a shared mailbox is added to Outlook, Outlook downloads the entire shared mailbox into that local copy by default. With a large shared mailbox, this can make the cache huge — slowing Outlook down or causing it to freeze for that person.

These steps apply to classic Outlook for Windows. The new Outlook for Windows and Outlook on the web work directly from the server and do not keep a local cache, so these settings do not apply there.

Tip 8. Turn off Download Shared Folders

This is the main fix. It stops Outlook from copying the shared mailbox onto the local computer. Outlook still shows the shared mailbox and works normally — it just reads from the server instead of from a local copy.

For a step-by-step walkthrough with screenshots, see Turning off Cached Exchange Mode for Office 365 shared mailboxes.

Tip 9. Keep less email offline

Outlook can keep less of your own email stored locally, which also keeps the cache smaller. Everything stays on the server and is still searchable.

1. Go to File, then Account Settings, then Account Settings again.
2. On the Email tab, select your Microsoft 365 account and click Change.
3. Find the slider labelled Download email for the past.
4. Move it to a shorter period, such as 3 months or 1 year.
5. Click Next, then Done, and restart Outlook.

Tip 10. Clear the old cache once

After you turn off Download Shared Folders (Tip 8), any shared mailbox content Outlook already downloaded stays in the local cache until the file is rebuilt. Rebuilding it once reclaims that space. This is a one-time cleanup — with the setting off, the cache will not fill up with shared mailbox email again.

1. Close Outlook completely.
2. Press Windows + R, type %localappdata%\Microsoft\Outlook, and press Enter.
3. Find the file ending in .ost and rename it by adding .old to the end.
4. Reopen Outlook. It builds a fresh cache that no longer includes the shared mailbox folders.

The .ost file is safe to rename because it is just a local copy of what is on the server. Never rename or delete a .pst file — a PST can hold the only copy of email and deleting it would lose data permanently.

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