When Outlook PST file is getting full (the maximum file size is 50 GB for Outlook 2010 and later versions), you may experience the following issues:

• Unable to send/receive new emails.
• Outlook becomes slow or unresponsive.
• PST file becomes corrupted (which can lead to data loss).

You can view your current PST file size at Tools > Mailbox Cleanup > View Mailbox Size.

To address the issue of a large PST file, you have a few options as follows.

NOTE: Please backup your PST file before proceeding with any of the following solutions.

Delete any unwanted or old emails to clear up the space. Remember to clear the Trash folder as well.

Outlook provides a feature to compact the PST file, which helps to reduce its size by eliminating unused space and optimizing the file structure. Please see the related guide at: Reduce the size of your mailbox and Outlook Data Files (.pst and .ost)

One effective way to reduce the size of an Outlook PST file is by saving email attachments to a different folder on your computer or network drive, rather than keeping them within the PST file itself.

To save attachments outside of Outlook and remove them from the PST file, you can follow these steps:

1. Open Outlook and select the email with attachments that you want to save outside of Outlook.
2. Click the drop-down arrow to the right of the attachment icon and select “Save All Attachments…”.
3. Once the attachments have been saved, you can delete the attachment by selecting “Remove Attachment” to free up space.

You can consider splitting a large PST file into multiple smaller PST files.
See the guide: How to Create Local .pst files in Outlook and move emails/folders by SOM Information Technology

This option is not recommended as large PST files are prone to corruption. If this option is necessary, please refer to: How to configure the size limit for both (.pst) and (.ost) files in Outlook