Hi,

I'm trying to resolve an issue for a client.

Their machine is Windows 10 64 bit.

They are using Outlook 2013, but from what I can see their IT recently installed office 365, and started using onedrive.

They are configured with an exchange account, and apparently they've been receiving .NET errors over last few days, I've also noticed a whole suite of AVG tools running in the background.

They are getting the error, "you are using an old .ost file, please delete and close outlook etc".

They were also unable to send mail, they said at the time, and PC was blue screening.

Whe I remoted in, I could see disk usage at 100% for at least 30 mins, it seemed there was an office process taking up 400mb of memory. The computer was completely unresponsive.

I left it for a couple of hours, then remoted back in. Seems to have finished whatever it was doing. I then logged into outlook to get the same "old .ost" error.

I looked in c:\users\username\app data\roaming\microsoft\outlook

But there were no .ost files

Looked in c:\users\username\app data\local\microsoft\outlook

and found several .ost files (some 5/6gb). I found the person I was looking for and renamed it to .bak - then restarted outlook.

This time, it still popped up with the error, but then said it would resync from the server, and started to download the .ost file again. I could see the file being recreated in the outlook folder (.tmp file) - and when finished it changed back to an .ost file.

I then restarted outlook, but keep receiving the error "using old .ost file". It's driving me mad.

They use a separate IT company, so I don't have all the mail setting, passwords etc. And the last thing they need is to start a fresh, they have so many custom folders, todo lists all sorts - I cannot lose their existing mails, contacts, folders etc.

Anyone know a simple way to fix this?

I can always remote in to check settings, if anyone has any questions.

Hope this forum can help, and Hi everyone!