If you have an old PST backup but only use Outlook on the web, you cannot directly open the PST file in your browser. A PST file stores Outlook data, including emails, attachments, and folders, locally on your computer. To access these emails online, you need to import the PST data into your Microsoft 365 mailbox using a supported method. If your goal is to open PST File in Office 365, migrating the archived mailbox data to your cloud account allows you to access the imported emails through Outlook on the web without relying on the original PST file.

The Aryson PST to Office 365 Migration Tool provides an alternative for transferring old PST backup data to a Microsoft 365 mailbox. You can load your PST file, select the required mailbox data, and connect to the destination account using the supported authentication settings. Once the migration is complete, sign in to Outlook on the web and verify that your emails, attachments, and folder structure are available in the destination mailbox. This method is useful for users who want to access archived Outlook emails online and manage their old mailbox data through Microsoft 365.