export/import mailbox feature is one of the best tool for any IT Admin.
If you are into a situation where you need to backup the old mailboxes then you can use this tool rather configuring the Oulook Client and making PST.It is very easy to use.

Things to remember - You should have Manage Full Access Permission on the desired mailbox, else it won't work.

 Export-Mailbox -Identity <MailboxIdParameter> -TargetMailbox <MailboxIdParameter> -TargetFolder <Folder_Name>