Depending on what you're asking here, you might now like the answer. If by "previous workplace" you mean another separate employer there probably is no way for your user to get access to their old email. This is less a technical thing and more standard policy most corporations have regarding email.
However, if you user *does* have a .pst file containing their email, then you can easily view that in Outlook (but I would find that to be unlikely). If you do have one open it in Outlook by using File -> Open -> Personal Folders File (.pst).
If, on the other hand, you are asking how does a user open another mailbox on the *same* Exchange server or Exchange site, then that is also easy. If you only need into the Inbox (or one of the other top level default folders, such as Calendar), then the easiest method is to goto File -> Open -> Other Users Folder. In Name enter the name of their old mailbox and on the pull down what you are trying to open. Of course the user will need tohave permissions to whatever folder you try to open there.
If you need access to more than the root Inbox (for example a sub folder from Inbox), then you will need to create another profile for Outlook. Repeat the same method you used to create the first profile but switch out the names.
-Heath
