Outlook OST Rebuild Fix Sync Problems and Missing Mail
New mail shows on your phone but never lands in Outlook, folders disagree with webmail, and Send/Receive crawls. For IMAP and Microsoft 365 accounts the OST cache is the usual suspect - and rebuilding it is both safe and simple, if you know the rules.
✍️ Technical Anshu📧 Microsoft Outlook⏱️ 15 minutes plus sync time
🎥 Watch the full process in the video - Outlook setup, profiles and every error fix, step by step
The OST is Outlook's local mirror of your server mailbox for IMAP, Exchange and Microsoft 365 accounts. When that mirror cracks - after crashes, disk errors or years of growth - Outlook syncs against a damaged copy and produces missing mail, stuck folders and phantom items. Rebuilding means deleting the mirror and letting Outlook download a fresh one from the server, where the real mailbox lives.
Status bar in OutlookDisconnected - This folder was last updated on 12-03-2026
The two things to check - the OST location in Data Files, and a status bar that admits the folder has stopped updating
1
First Confirm It Is Safe for Your Account Type
The whole trick depends on one fact: for IMAP, Exchange, Microsoft 365 and Outlook.com accounts, the mailbox lives on the server, so the OST is disposable. Check your account type under File > Account Settings:
IMAP / Exchange / Microsoft 365: safe to rebuild - everything resyncs
POP: stop. POP mail lives in a PST, not the OST. Deleting files will not fix POP sync because there is no server copy to rebuild from. Use the ScanPST and profile guides instead
One caution even for IMAP: any folders labelled On My Computer only exist only in the OST. Export them to a PST before rebuilding
2
Rename the OST Instead of Deleting It
Renaming gives you an instant rollback if anything surprises you:
Close Outlook completely and confirm in Task Manager that OUTLOOK.EXE is gone
Press Windows + R, type %LOCALAPPDATA%\Microsoft\Outlook and press Enter
Find your account's .ost file and rename it to .ost.old
Open Outlook. It announces it is preparing and creates a brand-new OST, then downloads the mailbox fresh
3
Let the Resync Finish Properly
Watch the status bar - it should move from Updating folders to All folders are up to date
A big mailbox on slow internet can take hours. Leave Outlook open and the PC awake; closing mid-sync is how partial mirrors happen
Check the webmail inbox count against Outlook afterwards - they should match
Once a week of normal use passes, delete the .ost.old file to reclaim the space
4
Reduce the Mail Kept Offline If It Breaks Again
If the OST keeps corrupting, it may simply be too large. Shrink the mirror:
File > Account Settings > Account Settings, double-click the account
Drag the Mail to keep offline slider down - 1 year suits most people; older mail stays searchable on the server and in webmail
Restart Outlook and let it tidy the file. For a dramatic shrink, rebuild the OST once more after changing the slider
5
If Sync Still Fails After a Fresh OST
A clean OST that still will not sync means the problem is not the cache:
Test the same account in webmail - if the server itself is wrong there too, it is a provider issue
Create a new Outlook profile and add the account there - profile damage mimics sync failure perfectly
Check the account is not stuck in Work Offline mode (Send/Receive tab) - a embarrassingly common cause
Temporarily disable antivirus mail scanning to rule it out
🔴 Never delete an OST on a POP account expecting a rebuild, and never treat the OST as a backup. If losing the server copy would hurt, export important folders to a PST - the OST is a mirror, not a vault.
🎥 Watch the Complete Process in the Video
Account setup, IMAP and POP settings, profiles, ScanPST - the full Outlook guide with live demo, step by step.
For IMAP, Exchange and Microsoft 365 accounts, no - the mailbox lives on the server and downloads again. The only exception is folders marked On My Computer only, which exist solely in the OST. Export those to a PST first.
How long does the OST resync take
It depends on mailbox size and connection speed. A 2 GB mailbox on decent broadband is usually under an hour; a 25 GB archive can take most of a day. Keep Outlook open and watch for All folders are up to date in the status bar.
My OST file is huge but my mailbox is small - why
OST files grow with history and do not shrink automatically when you delete mail. Rebuilding after reducing the Mail to keep offline slider is the reliable way to reclaim the space.
This guide is based on the Outlook Full Setup and Error Fix video by Technical Anshu • Stuck on any step, ask in the video comments and the community will help you 🙏