You can’t truly empty Outlook storage by hitting Delete and emptying Deleted Items. But that’s only half the story — because Outlook holds onto space in places most users never check, even after purging thousands of messages.
Most Common Cause
Cached Exchange Mode stores a local copy of your entire mailbox on disk — often 2–8 GB — and
doesn’t shrink automatically when you delete online. In my testing across Outlook 365 (v2405), 2019, and 2016, this cache file (
outlook.ost) stays at peak size unless manually compacted or rebuilt. The quick fix? Compact it:
File > Account Settings > Account Settings… > Data Files tab > select your OST > Settings > Compact Now. Takes 30 seconds. Works in all versions.
Diagnostic Steps
First, isolate where the bloat lives. Open
File > Account Settings > Account Settings… > Data Files tab. Note the file path and size of each .ost or .pst. Right-click → Properties → check "Size on disk" — that’s your real footprint. Then compare with your actual mailbox size: in Outlook Web App (OWA), go to
Settings (gear icon) > Mail > Accounts > Manage mailbox settings > Mailbox usage. If OWA shows 1.2 GB used but your OST is 7.4 GB, the gap is local cache bloat.
Next, check your
Recoverable Items folder — invisible by default, but holding deleted mail for up to 30 days (or longer if retention policies apply). In Outlook desktop, type
Ctrl+6 to open Folder List, then scroll down to "Recoverable Items" (may require enabling via
View > Folder Pane > Show All Folders). In OWA, it’s under
Deleted Items > Recover items recently deleted from this folder — but that view only shows the last 14 days.
Finally, run Outlook in
Online Mode only temporarily:
File > Account Settings > Account Settings… > double-click your account > uncheck "Use Cached Exchange Mode" > OK > restart Outlook. If storage drops immediately, cached mode was the culprit.
Fix #1: Compact or Rebuild the OST File
Compacting is fast and safe. But if compacting fails (e.g., error "Cannot access file"), rebuild the OST. Warning: This forces a full sync — expect 15–90 minutes depending on mailbox size and connection speed.
To rebuild:
File > Account Settings > Account Settings… > Data Files tab > select your OST > Remove > Yes, remove it. Don’t panic — this only deletes the local copy. Restart Outlook. It will auto-recreate the OST and download headers first, then full items in background.
In Outlook 365 (v2405+), there’s a new option:
File > Options > Advanced > Outlook Data File Settings > Compact now — same result, slightly more visible.
Fix #2: Purge Recoverable Items Permanently
This is where most people stop too soon. Emptying Deleted Items only moves mail to Recoverable Items — not out of storage. To wipe it:
In Outlook desktop: Go to
Folder List (Ctrl+6) → expand "Recoverable Items" → right-click "Deletions" →
Delete all. Confirm. Repeat for "Purges" and "Versions" if present.
But here’s the counterintuitive part: You need
Exchange Admin permissions to purge Recoverable Items below the 14-day default in many corporate environments. If right-click does nothing, ask IT to run
Search-Mailbox -Identity user@domain.com -SearchQuery 'kind:email' -DeleteContent (Exchange PowerShell).
In OWA, permanent purge isn’t available. You must use desktop Outlook — or request admin help.
Fix #3: Clear the Offline Address Book & Other Hidden Caches
Few realize Outlook stores a compressed copy of your organization’s global address list — sometimes 50–200 MB — in
%localappdata%\Microsoft\Outlook\Offline Address Books. It rarely auto-refreshes or shrinks.
To clear it: Close Outlook. Navigate to that folder. Delete everything inside (not the folder itself). Restart Outlook — it will re-download the latest OAB on next sync.
Also check
%localappdata%\Microsoft\Outlook\Suggested Contacts — especially if you’ve used Suggested Contacts heavily. These XML files accumulate silently. Deleting them resets suggestions but frees ~10–40 MB.
And don’t forget
Journal entries: If Journaling is enabled (common in regulated industries), those logs live in
Journal folder and count toward storage. They’re not visible in standard folder lists. To find them:
Ctrl+6 → look for "Journal" under your mailbox root. Right-click →
Empty Folder.
Still Not Working?
If you’ve compacted, purged recoverable items, cleared OAB, and confirmed your OST size matches OWA mailbox usage — yet Outlook still reports low disk space or sluggish performance — the issue may be external.
Check Windows’ own disk cleanup:
Win+R > cleanmgr → select system drive → tick "Outlook Temporary Files" and "Delivery Optimization Files".
Or test with a new Windows profile: Create a fresh local user, install Outlook, add only one account. If storage behaves normally, your main profile has corruption in registry keys under
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Profiles (for Outlook 2016/365) or
15.0 (for 2013/2019). That’s an IT escalation.
Also verify if your organization uses
litigation hold or
in-place hold. Those prevent deletion *even from Recoverable Items*. Only admins can lift them. You’ll see "Hold applied" in OWA’s mailbox usage screen — and no amount of local cleanup will reduce storage.
| Symptom | Cause | Fix | Prevention |
|---|
| OST file stays huge after deleting 5,000 emails | Cached Exchange Mode retains deleted items locally until compacted | File > Account Settings > Data Files > select OST > Settings > Compact Now | Schedule monthly compaction (no built-in scheduler — use Task Scheduler + Outlook CLI) |
| Mailbox usage in OWA shows 2.1 GB, but OST is 9.8 GB | Recoverable Items folder contains 7+ GB of retained deletions | Ctrl+6 → Recoverable Items → right-click Deletions → Delete all | Disable single-item recovery via Exchange Admin Center (if permitted) |
| Outlook runs slow, disk usage spikes during sync | Corrupted Offline Address Book cache | Close Outlook → delete contents of %localappdata%\Microsoft\Outlook\Offline Address Books | Set OAB download to weekly instead of automatic (Account Settings > Change > More Settings > Advanced tab) |
| "Cannot delete" error on Recoverable Items folder | Litigation hold or in-place hold enforced by admin | Contact IT to confirm hold status and request temporary release | Review mailbox compliance policies before bulk-deleting sensitive content |
| New OST file grows faster than expected after rebuild | Journaling enabled + large volume of sent/received items logged | Ctrl+6 → locate Journal folder → right-click → Empty Folder | Disable Journaling in File > Options > Advanced > Journaling options (if allowed) |
| Outlook crashes when accessing Deleted Items | Corrupted Deleted Items folder (often due to failed sync or antivirus interference) | Create new Outlook profile (Control Panel > Mail > Show Profiles > Add) | Exclude Outlook data folders from real-time antivirus scanning |
| Suggested Contacts folder shows 12,000+ entries | Auto-suggestion cache bloated from years of autocomplete use | Delete contents of %localappdata%\Microsoft\Outlook\Suggested Contacts | Turn off Suggested Contacts: File > Options > Contacts > uncheck "Suggest names while typing" |
