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: no, nothing is lost, and you don’t delete anything by hand. Here’s exactly what happens.

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 (instant reclaim, safe). Because the OST is only a cache, you can delete it and Outlook builds a fresh, smaller one:

  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 it to .ost.old (or delete it).
  4. Reopen Outlook — it recreates the OST and downloads only the 1-year window, so the new file is much smaller.
  5. Once mail looks complete, delete the .ost.old.

  No data loss: everything is re-downloaded from the server.

FAQ

Will I lose emails older than the window? No — they stay on the server forever (until you delete them in the mailbox). The slider only changes what’s cached locally for offline use.

Why didn’t my OST shrink after I moved the slider? Outlook removed the items but hasn’t compacted the file yet. Give it time, or rebuild the OST (above) to reclaim the space immediately.

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