Avatar of tech53
Flag for United Kingdom of Great Britain and Northern Ireland asked on

Outlook 365 keeps randomly corrupting OST file

Hi Experts,

I keep having this recurring problem with OST files for just one user.

This user has Outlook 365 and several O365 mailboxes configured.  Every now and then, he'll get an error message saying the OST file is corrupt and to run the repair tool. Now it is 60+GB in size plus he has access to several other shared mailboxes - all of which I'm sure are tipping into the one OST file. I have had to apply the registry entry MaxLargeFileSize to take it up to 100GB limit. I know this is never recommended. 

Sometimes, scanpst fixes it or rebuilding the OST file. It sits happy for a week or more then happens all over again.

So far I have tried:

- letting the scanpst tool complete repairing the file (fixes the problem but it re-occurs)

- deleting OST and rebuilding (again, a temporary fix - have done this quite a lot)

- new outlook profile

- Office repair, removal & reinstall

- disabling AV

- cleared out temp files

- disabling outlook addins

- completely removing the exchange mailbox and creating a new one from scratch

It's baffling.  I've seen it happen to another user on the same tenant - they too have a large mailbox but It only happened the once and a new outlook profile fixed it.

Any assistance would be great folks,


OutlookMicrosoft 365Microsoft Office

Avatar of undefined
Last Comment

8/22/2022 - Mon

If you have enough bandwidth and fast enough internet, forget about the OST, and only use Live/non-Cached Exchange mode.

Yeah, I've offered that too but the delay is a little too much.

Vasil Michev (MVP)

View this solution by signing up for a free trial.
Members can start a 7-Day free trial and enjoy unlimited access to the platform.
See Pricing Options
Start Free Trial
Ask your own question & get feedback from real experts
Find out why thousands trust the EE community with their toughest problems.

I think it might be interesting to see how big the file gets right before it fails. Maybe a small batch script that just runs in a loop in a minimized window that does two loops:

        dir("the .ost file") >> ostdir.log
        Sleep for 60 seconds

That would basically create a running log file that recorded the file size every 60 seconds. Once the corruption occurs, cancel the script and check the logs to see if there's any odd activity with it (e.g. a sudden spike in size then a drop).

Also, it's worth asking the user if they ever just have to shut down the PC or reboot with the power buttons on the computer, or if they ever lose power (e.g. battery on a laptop dies before being plugged in), or if they ever pull the power cable or turn off the power strip, or if they have any sleep or hibernation settings active.

I'm hoping that the ost file isn't somehow part of the synced data on a roaming profile. 

Just trying to think of different events that could potentially interrupt the process of writing data to the file, leading to corruption.

I wholeheartedly agree that this user should be using archiving of some kind. A 60 gb ost should only exist in stories told by admins around a campfire.
Experts Exchange is like having an extremely knowledgeable team sitting and waiting for your call. Couldn't do my job half as well as I do without it!
James Murphy

If the whole company has this problem (not just corrupt OST, but maybe just slow Outlook, and search problems):


This will help you beat both. Searching is amazing (including INSIDE PDF/Word/Excel), and you offload email from the MS Cloud, and your backup is extremely portable (in case of disaster, restoring the externally saved backup takes mere minutes). Just a couple of hundred bucks for any mid-sized company (50 users) per year. Just buy any decent big storage server, doesn't have to be too powerful.

Thank you all.  Very well explained and I am 100% in agreement. OST files should not reach that level.  I think I'm going to have to apply discipline and just enforce archiving.