Outlook cached mode and OST files
How classic Outlook caches mailbox data locally — what an OST file is, when to use cached mode, and the modern alternatives.
For classic Outlook for Windows, cached mode is the local-cache-of-the-mailbox model that has been standard for decades. With cached mode enabled, Outlook downloads mailbox content into a local file (.ost) and works against that, syncing with Exchange Online in the background. With it disabled, Outlook talks live to Exchange for every operation. The choice matters more than it used to.
What cached mode does
In cached mode, Outlook:
- Downloads a local copy of the mailbox into an
.ostfile (usually in%LOCALAPPDATA%\Microsoft\Outlook\). - Reads and writes against that local copy first.
- Syncs changes to and from Exchange Online over the background.
- Provides snappy UI even on slow networks.
- Allows offline reading and composing.
Cached mode is the default for new Outlook installs.
When cached mode is right
- Most knowledge workers on personal devices benefit from the speed and offline capability.
- Mobile users who experience network variability.
- Users with large mailboxes where waiting for live-mode response on every click is painful.
When cached mode causes problems
- Very large mailboxes (>50 GB) can produce very large
.ostfiles, slow to download initially and prone to corruption. - Shared mailboxes with hundreds of GB of data — auto-mapped to a user's profile in cached mode, the
.ostcan balloon. - VDI / shared workstations — the
.ostdoesn't survive between sessions, so it re-downloads constantly. - Disk-constrained devices — laptops with small SSDs may struggle.
Cached mode tuning
Group Policy and Intune configuration can:
- Limit cached time — only cache the last 1, 3, 6, 12 months instead of everything.
- Disable shared mailbox caching so they're live-only.
- Disable cached mode entirely for VDI scenarios.
For shared mailboxes specifically: turn off CacheOthersMail for any user mapped to a multi-GB shared mailbox. They access it live instead, with much better performance.
The new Outlook for Windows
The new Outlook for Windows (replacing classic Outlook over the coming years) takes a different approach: it's a thin web client that talks to Exchange via Microsoft Graph, with much smaller local cache footprint. Cached mode and .ost files are no longer the model. For most users, this works well.
The trade-off: heavy users who relied on classic Outlook's behaviour offline (long flights, intermittent connectivity) may notice differences. Microsoft has been adding offline support to the new Outlook progressively to close the gap.
What to do today
- Classic Outlook with cached mode: still fine for traditional knowledge workers. Tune retention and shared-mailbox caching as needed.
- VDI / cloud PC users: disable cached mode or use new Outlook with cloud-cache features.
- Planning ahead: new Outlook is the strategic direction; pilot it now to find any classic-only dependencies.
.ost files aren't going extinct soon, but their day-to-day relevance is shrinking.