Outlook OST File Too Big? How the Cache Slider Works — and Whether Old Mail Gets Deleted

Your Outlook .ost (the offline cache for a Microsoft 365/Exchange account) has ballooned to many GB and is filling the disk. You want to set “Mail to keep offline” to 1 year (or 6 months) — but first, the question everyone has:

If Outlook already downloaded everything, and I now set the slider to 1 year, does it delete my old email? Do I have to delete something manually?

Short answer: the slider does not delete older mail from the server. You do not need to delete the OST to change it. Before clearing or rebuilding any cache, preserve items that have not synced to the server.

What the slider actually does

The “Mail to keep offline” slider (File → Account Settings → your account → Change → the slider: 1/3/6/12/24 months or All) controls only the local cache, not the server:

• When you shorten the window, Outlook automatically removes the older items from the local OST the next time it syncs. You do not delete anything manually.
• Nothing is deleted from the server. Those older emails still live in your mailbox on Exchange/Microsoft 365. When you scroll to the bottom of a folder you’ll see “Click here to view more on Microsoft Exchange” — click it (while online) and the older mail is right there.

So setting it to 1 year just means “keep the last 12 months on this PC; fetch anything older from the server on demand.”

How to change it

1. File → Account Settings → Account Settings → double-click your Exchange/365 account.
2. Move “Mail to keep offline” to 1 year (or 6 months).
3. Next → Done, then restart Outlook.
4. Leave Outlook open and online for a while — it re-syncs and trims the cache to the new window automatically.

Reclaiming the actual disk space (the part people miss)

Here’s the catch: after Outlook removes the old items, the .ost file on disk often doesn’t shrink right away — Outlook frees space inside the file but the file’s physical size can stay large. Two ways to actually get the gigabytes back:

• Let it settle / restart a couple times. Outlook compacts the OST in the background over time; the file size drops on its own.

• Rebuild the OST after preserving local-only items. First compare Outlook with the web mailbox, including mail, calendar, contacts, Drafts and Outbox. Export any items not on the server to a PST using File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst), and verify the export. An OST can hold unsynced changes; rebuilding before exporting can lose them. Then:

  1. Close Outlook.
  2. Go to %LOCALAPPDATA%\Microsoft\Outlook and note the large .ost file (or find its exact path at Account Settings → Data Files).
  3. Rename the correct Exchange account’s file to .ost.old and keep it while checking the rebuild. A renamed OST is not a substitute for the PST export.
  4. Reopen Outlook — it recreates the OST and downloads only the 1-year window, so the new file is much smaller.
  5. Check the rebuilt mailbox against the web and your exported local-only items before removing the old file. Restore missing exported items if needed, avoiding duplicates.

  Only server-synced data is automatically re-downloaded. Rebuilding takes time and needs enough space for the new cache as well as your preserved data.

FAQ

Will I lose emails older than the window? The slider does not delete them from the server. They remain subject to the mailbox’s retention policies and any deletion or archiving you or your organization performs.

Why didn’t my OST shrink after I moved the slider? Outlook may not have compacted the file yet. Give it time; if rebuilding is necessary, preserve unsynced items first and allow time for the new cache to download.

Is 50 GB really the OST limit? Cached Exchange Mode has a large default size limit, but a giant OST hurts performance long before that. A 6–12 month window keeps it lean.

Does this affect a POP/IMAP .pst? No — this is about Cached Exchange Mode .ost files. The “keep offline” slider only appears for Exchange/Microsoft 365 accounts.

Sources: Microsoft Learn — Plan and configure Cached Exchange Mode (sync slider), Microsoft Support — Turn on Cached Exchange Mode

Additional sources: Microsoft — preserve unsynced items before OST deletion