Forum Discussion
why do I have two OST files with constantly the same exact size?
I am using Microsoft Outlook 365 on Windows with multiple email mailboxes. One mailbox is a microsoft hosted mailbox with a custom domain name (lewins.email) and another is a gmail hosted mailbox (GSDVS.COM) which is accessed from inside Outlook using IMAP.
In my data files directory I have two OST files, one for each mailbox and they have almost exactly the same date/time stamp (which I guess is not strange since they are both being constantly updated by Outlook) but also the exact same size down to the byte. Why is that? Shouldnt the size be different? Isnt something wrong with how my outlook is setup?
Some screenshots below, feel free to ask if you need more info about how each mailbox is setup.
Thanks for any advice you can give.
4 Replies
Something is configured incorrectly, Exchange OST and IMAP OST do not talk to each other. I would review the data on each server to see what the mailbox size is, one or both might be off. This can happen with a corrupted profile. I recommend backing up both to a PST first, then make a new profile and let all the email download. Specify how much data you want to download in each account, 1 year for Exchange is default and IMAP is unlimited. You can change both. That should change the OST to be accurate.
I was hoping to avoid losing my profile but your suggestion will at least help me resolve the issue. Thank you.
You can have more than one profile, so I wouldn't delete the one that is having corruption, just leave it and make the new one default when opening Outlook.
1. Rapid problem determination
Self-checking steps:
Open File Explorer → Enter the path:
%localappdata%\Microsoft\Outlook
Check for the existence of two .ost files (e.g. Outlook1.ost and Outlook2.ost).
Compare the modification date and file size of the two files.
2. Common Causes
Possible scenarios:
Duplicate account configurations: the same mailbox account has been added multiple times
Possible scenario: Duplicate account configurations: The same mailbox account has been added multiple times.
Migration legacy: left over from the old version or when changing computers.
3. Security solutions
Method 1: Delete redundant OST files
Close Outlook (make sure the process exits completely)
Delete older or unused OST files (backup is recommended)
Restart Outlook, the system will automatically rebuild the cache.
Method 2: Rebuild the Outlook profile
Open Control Panel → Mail → Show Profiles
Delete the current profile → Create a new profile and re-add accounts
Method 3: Disable cache mode (temporary)
File → Account Settings → Double-click the mailbox account
Uncheck “Use cached Exchange mode”
Re-enable the feature after application
4. Preventive measures
Avoid recurrence of the problem:
Regularly clean up the OST file (select “Outlook Cache” via Disk Cleanup utility)
Avoid logging in to the same account on multiple devices at the same time
5. Advanced check (optional)
Use the ScanPST utility:
Run SCANPST.EXE (path is usually C:\Program Files\Microsoft Office\root\Office16)
Scan and fix possible mailbox data errors
