Please follow the below guide if you think you need (or have been told to) clear down the size of your mailbox.Option
Method 1:
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Disable shared folder synchronization
Shared Folders can also quickly increase the OST file to reduce the size.
Outlook has the option to compact the OST and PST files to reduce their size. The steps are as follows:
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size and cause Outlook performance issues. To resolve this, disable Shared Folders synchronization by following these steps:
Click File > Account Settings > Account Settings.
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Under the the Data Files tab, select the OST data file that you want to compact.
Click Settings> Advanced > Outlook Settings > Advanced > Outlook Data File Settings…
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Click Compact Now and then click OK.
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This will compress the OST file to an extent and may resolve the issue temporarily.
Option 2: Clean up the mailbox
To fix the issue for a long time, To reduce the OST file size, you can choose to clean up your mailbox. However, this method requires you to delete older emails. The steps are as follows:
Click the File> Info and click on Cleanup Tools beside Mailbox Cleanup
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Click on Mailbox Cleanup… option.
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You can view your current mailbox size stored in the local OST file and on the server by clicking on the ‘View Mailbox Size…’ button.
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Enter the number of days to find the older emails and delete them to reduce the size of the OST file and mailbox on the server.
You can also Find items larger than, the Empty Deleted Items folder, View Deleted Items Size, View Conflicts Size, or Empty Conflicts to reduce the OST file size.
Further, you can look for large emails, download all the attachments, and then remove the attachments from emails to reduce the OST file size.
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Uncheck the Download Shared Folders option and click OK.
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Restart Outlook and wait for synchronization to finish.
After you make this change, none of the shared folders in the secondary mailbox will be available to you in Outlook when you are working offline. However, all folders in the primary mailbox will still be available to you when you are working offline.
Method 2: Delete large Outlook emails and attachments
Deleting large emails, undoubtedly, is the wisest technique. Rather than randomly deleting large-sized files, you can follow a systematic approach to manage Outlook attachments . The steps are:
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Open Outlook. In the folder list, click Search folder, then select New Search folder.
Scroll down the list and under Organizing Mail, select Large mail. Click Choose under Customize Search Folder.
Specify a size in the ‘Show mail greater than’ box (for example 6000 KB). Click OK to continue.
Click OK again and a folder named ‘Larger Than 6000 KB’ is created under Search Folders. This newly created folder will get updated with the mails of size more than 6000 KB automatically.
Method 3: Compress the outlook PST data file
In Outlook, Outlook data files can be compressed manually. Compressing files would help in restoring some space in the mailbox. Before starting the procedure, delete the items that are not required. In Navigation pane, right-click the Deleted Items folder and then click Empty ‘Deleted items’ folder. Then follow the steps given:
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Launch Outlook. In the File tab, select Account Settings and then click Account Settings.
Click the Data Files tab, select the PST file to compact, and then click Settings.
In Outlook Data File dialogue box, click Compact Now. This is the final step.