Outlook has a limit on the size of the offline Outlook Data File (.ost), which is used for IMAP, Microsoft 365, and Outlook.com accounts. When the .ost file reaches this limit, you may experience performance issues and you might not be able to add more data to the .ost file.
Here are some steps you can take to resolve this issue:
Archive Old Emails: Archiving old emails can help reduce the size of your mailbox and free up space. You can set up automatic archiving in Outlook to move older items to an archive folder.
Compact the PST File: Compacting the PST file can help reduce its size. To do this, right-click the PST file in Outlook, select ‘Data File Properties’, click ‘Advanced’, and then click ‘Compact Now’.
Delete Unwanted Emails: Deleting unwanted emails, emptying your junk email folder, and deleting items from other folders can also help reduce the size of your mailbox.
Increase the Outlook Size Limit via Registry: You can increase the size limit of both the .pst and the .ost files in the Windows registry. However, increasing the file size limit should be done with caution as it can impact the performance of Outlook and your system.
Use Mailbox Cleanup tool: Outlook has a built-in Mailbox Cleanup tool that can help you manage the size of your mailbox. You can find it under File > Tools > Mailbox Cleanup.
Remember to always back up your data before making any major changes to your system. This can help prevent data loss in case something goes wrong during the process.