For archiving purposes run the following command in Exchange Management Shell to export a mailbox to a PST File and to save the file in the folder pst:

Get-Mailbox Testuser | New-MailboxExportRequest -FilePath \\servername\c$\pst\testuser.pst

The status is Queued. Wait a few minutes. Then check the status with

Get-MailboxExportRequest -Status Completed

More about exporting to PST in my German article Bulk Export von Exchange Mailboxen in eine PST Datei.