OST File bigger as shown in Outlook or online.

Hello somchaiq,

Good day!

Sorry for the delay!

For the E3 account with a 100 GB mailbox, if you have enabled Online Archive, for instance with an Office 365 E3 license, you can move the contents out of your mailbox and into your Online Mailbox to reduce the size of your mailbox and thus reduce the size of the ost-file as the Online Archive isn’t being cached locally.

For the MS Office Business standard with 50 GB mailboxes.

First You may check the status of the folder size with the command below: connect your PowerShell with Exchange Online. share the results with us. (hide your email address)

Get-MailboxFolderStatistics -Identity ******@domain.com -FolderScope RecoverableItems | fl Name, FolderPath, Identity, FolderSize

Microsoft Outlook mailbox increases as you receive emails and create items, thus making the OST file larger than the mailbox. However, if you look at the OST file size, you will find that it does not actually represent the mailbox size but it also stores many other items. This may result in the OST file being larger than the mailbox. It can be also related to :

1. Cached Exchange Mode: Outlook uses a cached Exchange mode by default, which downloads a local copy of the mailbox (OST file) to improve performance. This local copy can sometimes grow larger than the actual mailbox size.
2. Deleted Items Folder: Deleted items are retained in the mailbox for a certain period before being permanently deleted. If there are many items in the deleted items folder, they contribute to the mailbox size even though they may not be visible in Outlook.
3. Sync Errors: Sync errors or issues during synchronization can cause items to be replicated or not properly deleted, leading to a larger OST file size.

4-It can be possible to increase the maximum ost-file size via the Registry or Group Policy.

Here are a few steps you can take to troubleshoot and potentially reduce the OST file size:

1. Empty the Deleted Items Folder: Right-click on the Deleted Items folder in Outlook and select "Empty Folder" to permanently delete all items within the folder. Be aware that this action cannot be undone, so make sure no important items are stored in the Deleted Items folder before proceeding.
2. Purge Deleted Items: In Outlook, go to the "Folder" tab and select "Purge" from the "Clean Up" section. This will remove any items that are marked for deletion but haven't been removed from the mailbox.
3. Compact the OST File: In Outlook, go to the "File" tab and select "Options." In the Outlook Options dialog box, click on "Advanced" and under the "Offline Outlook Data File" section, select "Compact Now." This will compress and reduce the size of the OST file.
4. Disable Cached Exchange Mode (Optional): If reducing the OST file size is a priority, you can consider disabling the cached Exchange mode. However, disabling it means that Outlook will directly access the mailbox on the server, potentially affecting performance, especially in environments with slower or unstable internet connections.

5-Don’t cache Shared Folders and/or Public Folder Favorites.
When you have access to Shared Folders, like a Shared Mailbox or a mailbox of a manager or colleague, or configured Public Folder Favorites, then Outlook could be caching these as well. You can turn this of via;

File-> Account Settings-> Account Settings…-> double click on your Exchange account-> button: More Settings-> tab: Advanced-> disable: Download shared folders

6- **Reduce the amount of Shared Mailboxes.**When you have Full Access permissions to a mailbox (having Send As rights as well is recommended too), you can configure the mailbox as an additional account instead of a Shared Mailbox. This will give the mailbox its own ost-file as well, instead of caching it in the ost-file of the main account.

7-Adjust the Sync Slider. This allows you set how long emails should remain cached within the ost-file on your local computer.

File-> Account Settings-> Account Settings…-> double click on your Exchange account-> move the Sync Slider below: Download email for the past

8- Enable the archive online and Move Mailbox data to the Online Archive.

Keep in mind that even after taking these steps, the OST file may not immediately shrink in size. Outlook will gradually clean up and update the OST file over time as it synchronizes data with the Exchange server.

If the OST file size continues to cause issues, it may be necessary to archive or manually move older or less frequently accessed items to a separate Outlook data file (PST file) to further reduce the mailbox size.

We look forward to your response. This requested information will help us to narrow down the scenario and find the right troubleshooting. We will keep assist you based on the information you will provide. I sincerely appreciate your patience and cooperation.  Eben Ezer Tres | Microsoft Community Moderator

Hello somchaiq,

Thank you for your reply,

Based on the PowerShell result you have shared, it is a little bit strange your issue.

1-Please can you try to create a new profile and delete all of the old PST files to check the issue

2-Please can you try to use the affected account on another computer to check if you are still facing the same issue

3-In the meantime if the issue still perssist I recommend you create a support ticket online, they can use some back-end tool to synchronize the affected mailbox and do some diagnostics to find out the real issue.

please see below for instructions: 

1. Create a Service Request in the O365 Admin Portal and indicate the request as clearly as possible.
2. Or you can contact our hotline and we will create a Service Request ticket for you Global Customer Service phone numbers (microsoft.com)

We look forward to your response. This requested information will help us to narrow down the scenario and find the right troubleshooting. We will keep assist you based on the information you will provide. I sincerely appreciate your patience and cooperation.

Sincerely,

Eben Ezer Tres | Microsoft Community Moderator

Hello Eben Ezer Tres,

for the first case I can understand your suggestions, also I can tell you that I did this already.

The Mailbox at the Computer is just getting bigger as the email kept in the Inbox and sent items.

The second customers case is even more worse. Having only 3 GB in the mailbox and a 50 GB OST File. Telling me to reduce the size through deleting emails or archiving email is really not the right way to fix this problem. In addition his mailbox has a size of 100 GB and therefore the 50 GB maximum should not be related to his email account.

Here the results of the powershell command you sent earlier.

For the 40 GB mailbox

For the 3 GB small Mailbox

Right now another customer has the same problem, his three-month-old Mailbox should have 3.0 GB but the OST File is over 49 GB big. In this case, it should be not a problem since he has an E3 account with a 100 GB mailbox.

Hello somchaiq,

Good day!Thank you for posting to Microsoft Community. We are happy to help you.We are looking into your situation, we will update the thread shortly.Appreciate your patience and understanding and thank you for your time and cooperation.Sincerely,Eben Ezer Tres | Microsoft Community Moderator