To reduce the size of a PST (Personal Storage Table) file in Microsoft Outlook, you can try the following methods:

Archive Old Emails: Use the Archive feature in Outlook to move old or infrequently accessed emails to a separate archive PST file. This helps to reduce the size of the main PST file. To archive emails, go to the File tab, click on Options, select Advanced, and click on AutoArchive Settings. Configure the settings according to your preferences and run the AutoArchive process.

Compact the PST File: Outlook has a built-in feature to compact PST files, which removes empty or unused space and reduces the overall file size. To compact a PST file, go to the File tab, click on Account Settings, and select Data Files. Select the PST file you want to compact, then click on Settings and choose Compact Now.

Delete Unnecessary Items: Manually review your mailbox and delete any unnecessary emails, attachments, or other items that are no longer needed. This can help reduce the overall size of the PST file.

Empty Deleted Items Folder: The Deleted Items folder in Outlook can also contribute to the size of the PST file. Empty the Deleted Items folder regularly to permanently remove deleted items and free up space in the PST file.

Use the Mailbox Cleanup Tool: Outlook includes a Mailbox Cleanup tool that helps you identify and remove large or unnecessary items from your mailbox, including the PST file. To access the Mailbox Cleanup tool, go to the File tab, click on Options, select Advanced, and click on the Mailbox Cleanup button.

Split the PST File: If the PST file is still too large after trying the above methods, you can consider splitting it into multiple smaller PST files. This can be done using third-party tools or the built-in Import and Export feature in Outlook. Splitting the PST file allows you to manage your data more efficiently and prevent future size-related issues.

Greetings,
Peter