If your Outlook mailbox is full of old messages but you don’t want to lose them, Using the MSOutlookHelp OST to PST Converter, you can move Old Emails to PST File and store them outside your main mailbox. The tool keeps your original folder structure intact Inbox, Sent Items, Drafts, and custom folders remain organized as they were. It also supports exporting other mailbox items like contacts and calendars alongside your emails. This makes it easy to carry everything with you or store it as a long-term archive.

For more: https://www.msoutlookhelp.com/blog/move-old-emails-to-pst-file/