Outlook and huge OST file growing like crazy till reaching 50GB.

It has been few days since I was working on one case – OST file of one user was growing till 50GB at exponential speed despite mailbox had 5GB.

Case was really weird, what I did I added all of the shared mailboxes as additional accounts so I was able to turn off caching and put this particular mailbox in Online mode and recreated the profile – that didn’t help.

Well, I noticed that there were some sync issues (not much) – so I moved the mailbox to another database as move might omit some corrupted messages leaving user mailbox in healthy status in the target database – that didn’t help either.

So I took a completely separate computer, asked user to logged in there and enabled the cache mode – after weekend we had same issue.

I was like:


Then my colleague told me that there is possibility to check LOCAL size of the folders – not the server size. Because you need to know I ran Get-MailboxFolderStatistics against that user mailbox and I was adding size of each folder to check if maybe Get-MailboxStatistics is showing wrong values (yeah I know, that was silly).

And here is how to check this sizes:




When I opened this window in user’s Outlook I noticed that the overall size was 80GB! So I went through these folders, and guess what – I was already looking at this, I was like one click from resolving the mystery. When you go to the sync issues folder there os something called “Local failures”, these local failures does not contain error messages – folders contains whole messages with attachments and Outlook creates new item each time it tries to sync it with the server… And what I did was lookign at messages in Sync Issues (these are just pain text error messages) and as I didn’t see many “unread” messages at the local failures folder I haven’t even clicked on it. And apart from 29 messages with unread status, local failures contained hundreads of copies of just few messages with attachments. Yup…


Good thing is – resolved the problem! Bad thing is – user still gets many sync issues – working on that. Will post something about it once succesfully resolved.

Small update:

OK, bad thing is failure messages are still generated. Well.. Shit, will let know about outcome of further investigation.

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