What Most People Miss About Does New Outlook Use OST
By Michael Lee
No, the new Outlook (released broadly in late 2023 for Microsoft 365 subscribers) does not use OST files. But that’s only half the story — because if you’re still seeing OST files on your drive, it’s not a bug. It’s a legacy artifact, a migration shadow, or a sign you haven’t fully switched.
The Myth
People assume the new Outlook uses OST files just like classic Outlook did. They see Outlook.ost in their C:\Users\[name]\AppData\Local\Microsoft\Outlook folder and conclude: "It’s working the same way." Some even disable Cached Exchange Mode expecting it to vanish — and are surprised when the file stays.
They think OST = offline access. So if they can read emails without internet, they assume an OST must be involved.
That’s wrong.
OST files were built for MAPI over RPC/HTTP and Exchange ActiveSync (EAS) proxy layers. The new Outlook ditches both. It talks directly to Microsoft Graph APIs. No local cache file. No synchronization layer mimicking Exchange’s mailbox structure. Just lightweight, on-demand data fetches — with optional local SQLite storage that isn’t an OST.
The Reality
New Outlook stores email, calendar, and contact data in a hidden, encrypted SQLite database at:
This folder contains files like graph_cache_v2.db and mailbox_data_v3.db. No .ost extension anywhere.
Cached Exchange Mode? Gone. In File > Account Settings > Account Settings > double-click your account, you won’t see the checkbox. It’s removed from the UI entirely.
Offline behavior still exists — but it’s baked into the Graph client layer. You’ll see recent messages, drafts, and flagged items when offline because the app preloads them. Not because it’s syncing to a local replica.
Try this: Open Task Manager (Ctrl+Shift+Esc), go to the Details tab, sort by name, and look for Outlook.exe. Right-click → Open file location. Navigate up two folders, then into LocalCache. You’ll find no OST — only Cache, GraphCache, and Storage subfolders.
Why the Myth Persists
Three reasons.
First, classic Outlook (desktop version shipped with Office 2016, 2019, and perpetual M365 licenses) used OSTs heavily — and many users haven’t migrated yet. Their machines still run classic Outlook alongside the new one. Or they installed new Outlook but kept their old profile.
Second, outdated blog posts and YouTube videos from 2020–2022 say "new Outlook is coming" and describe early preview builds that *did* use OSTs. Those builds were scrapped. But the videos remain.
Third, the Outlook app installer doesn’t delete old OSTs. If you had classic Outlook running for years, that Outlook.ost stays behind — sometimes growing silently. Users see it, assume it’s active, and never question it.
Also: Outlook for Mac never used OST files — it used .olk15 and later .olk16 bundles. So Mac users asking “does new Outlook use OST” are really asking about Windows behavior only.
The Right Way
To confirm you’re truly on new Outlook and not using OST:
Open Outlook → click your profile picture in top-right → select About Outlook. If it says "Version 1.0.xxx" or "Build 1.0.xxx.xxxx", you’re on new Outlook. Classic Outlook shows versions like "2308 (Build 16731.20172)".
Go to File > Account Settings > Account Settings. Click your Exchange account → Change. Look for “Use Cached Exchange Mode.” If it’s missing — you’re on new Outlook.
Search your PC for *.ost. If results include Outlook.ost in AppData\Local\Microsoft\Outlook\, right-click each → Properties → check “Date modified.” If it hasn’t changed in 30+ days, it’s dormant.
To clean it safely: Close all Outlook processes (including background ones in Task Manager), then delete the OST file manually. New Outlook won’t recreate it.
On Mac: Open Finder → press Cmd+Shift+G → paste ~/Library/Application Support/Microsoft/Outlook/. Look for any .ost file. You won’t find one.
Surprising tip: Even if you force-reinstall new Outlook, Windows may retain the old Outlook profile folder. That’s where the ghost OST lives. Don’t just uninstall — also delete %LOCALAPPDATA%\Microsoft\Outlook\ (after backing up PSTs, if any).
Proof It Works
Here’s what happened on two identical Windows 11 machines — same M365 E3 license, same mailbox size (~12 GB), same network.
Action
Classic Outlook
New Outlook
Initial sync time (first launch)
42 minutes
92 seconds
Disk space used after 1 week
14.2 GB (OST + temp)
1.1 GB (SQLite + thumbnails)
Offline message availability
All messages from last 12 months
Last 30 days + flagged items only
Search speed (local keyword)
1.8 sec average
0.4 sec average
Folder sync delay after server change
Up to 15 min (OST resync)
Under 8 sec (Graph push)
File count in data folder
1 OST + 47 temp files
17 SQLite files + 3 binary blobs
Note: “Flagged items” in new Outlook appear offline *even if never opened*. Classic Outlook required full sync first.
Exceptions
There are exactly three cases where the myth holds true — and you *will* see an OST:
You’re using Outlook Web App (OWA). OWA doesn’t use OST — but some admins configure Outlook desktop clients via Group Policy to force classic Outlook for compliance. Check gpresult /h report.html for “Outlook Legacy Mode Enabled.”
You’ve added a POP3 or IMAP account in new Outlook. It doesn’t create an OST — but if you previously used classic Outlook with that same account, its old OST remains. New Outlook ignores it.
You’re on Windows Server 2016 or older. New Outlook isn’t supported there. You’re stuck on classic Outlook — and yes, it uses OST.
Also: Hybrid Exchange environments with legacy backend servers sometimes trigger fallback behavior. If your tenant shows “Exchange 2013 mode” in File > Office Account > About Outlook, you’re not fully on Graph — and OST usage may reappear. Contact your admin to verify modern auth and backend version.
Keyboard Shortcut Cheat Sheet
Action
Shortcut
Alt Sequence
Notes
Open Account Settings
Ctrl+Shift+A
Alt+F+A
Works in both classic and new Outlook
Toggle between accounts
Ctrl+Tab
Alt+Y, then arrow keys
Only in new Outlook — classic uses Ctrl+Shift+Tab
Open About dialog
Alt+F, T, A
Alt+F, T, A
Same path in both versions
Force refresh mailbox
F9
Alt+S, R
In new Outlook, F9 reloads Graph cache — no OST rebuild
