Step 2 - Save the contents of a former employee's mailbox
In this step, place a Litigation Hold or In-place Hold on the user or export their Outlook data to a .pst file.
Place hold or export user's data to a .pst file
Once you've blocked a user from being able to log into your organization you can save the contents of their mailbox. There are two ways you can save the contents of the former employee's mailbox.
Place a Litigation Hold or In-Place Hold on the mailbox before the deleting the user account. This is much more complicated than the second option but worth doing if: your Enterprise plan includes archiving and legal hold, litigation is a possibility, and you have a technically strong IT department.
After you convert the mailbox to an "inactive mailbox," administrators, compliance officers, or records managers can use In-Place eDiscovery tools in Exchange Online to access and search the contents.
Inactive mailboxes can't receive email and aren't displayed in your organization's shared address book or other lists.
To learn how to place a hold on a mailbox, see Manage inactive mailboxes in Exchange Online.
OR
Add the former employee's email address to your version of Outlook on Desktop, and then export the data to a .pst file. You can import the data to another email account as needed. Check out Step 4 - Give another employee access to OneDrive and Outlook data.