In the past users might have archived their mailboxes to create space. Now that users are given very large mailboxes we encourge them to import the existing psts into their Microsoft 365 mailbox. 

Visit the Microsoft Support website for instructions on how to Import email, contacts, and calendar from an Outlook .pst file

1. At the top of your Outlook ribbon, select File.

Note: What data is imported?

.Pst files are created when you export or back up email, contacts, and calendar to an Outlook .pst file. Outlook makes a copy of your email to the .pst file.

• When Outlook exports emails to a .pst file, it includes any attachments. So, when you import a .pst file, you'll see your attachments.
• Outlook does not export meta-data such as folder properties ( views, permissions, and AutoArchive settings); message rules; and blocked senders lists. So, they won't be imported, either.
• When Outlook exports email, contacts, and calendar info to a .pst, it makes a copy of the information. Nothing is deleted from Outlook. You'll still see and have access to your email, contacts, and calendar in Outlook.