In order to archive a mailbox to .pst file before deleting a user account follow these steps;
1. Run the following command using the Exchange Management Shell replacing administrator with the user you want to grant access.
New-ManagementRoleAssignment –Role “Mailbox Import Export” –User Administrator
2. Run the following to list all the mailboxes
3. Run to following command to export the mailbox. Note you need to use a UNC pathname
New-MailboxExportRequest -Mailbox %aliasname% %servername%c$pst%aliasname%.pst
4. Check the status by typing