Information: The Export-Mailbox cmdlet exports all message types, including messages, calendar items, contacts, distribution lists, journal entries, tasks, notes, and documents. However, the Export-Mailbox cmdlet does not export rules.

To export data from a .PST file, you must run the Export-Mailbox cmdlet on a 32-bit computer that has the 32-bit version of the Exchange management tools and Outlook 2003 Service Pack 2 (SP2) or Outlook 2007 installed.

Please be aware that there are prerequisites to exporting mailboxes.  You need a location to export the mailbox to  along with permissions set on the mailbox.

In the example below I will show you how:

First add permission to the users mailbox

Add-MailboxPermission -identity user Accessrights FULLACCESS -User admin

Now we export the mailbox

Export-Mailbox -identity user -PSTFolderPath c:user.pst

Now you should be safe to remove the email boxes and free up space