Thank you for confirming your version. Robert's suggestion of using Shared Resources is a good one because it allows you to share other resources like calendars, contacts, etc. However, this would use up a mailbox account that was no longer being used by an employee. There's another option you could do that would allow you to shut down the mailbox no longer in use, but, keep in mind, this method will only move emails - no other resources.
SmarterMail 13.x offers a Restore feature. This allows you to restore a user's account, individual email messages and mail folders. Using this feature, you could move the old employee's messages to another employee's account. I'll give you the basic steps on how this is done:
Go to the SmarterMail server files. Find the old user that has left the organization. We'll call this person Tom. In Tom's user folder, create a new folder called "Tom's Messages". Copy all of Tom's mail folders (Inbox, Junk E-mail, Sent, etc.) and paste them into this new folder. Now, move the Tom's Messages folder into the new user's folder. We'll call the user who is taking over the messages Joe. So in the system files, Joe's user folder should now show his mail folders, Inbox, Junk E-mail, Sent, along with Tom's Messages.
Now that the system files are in place, log into the SmarterMail Web interface as the system admin. Click Manage > Restore. Choose Attach Folder. Enter Joe's email address and, in the folder path, put "Tom's Messages". Click Restore. When you go to Joe's mailbox now, you should see a folder for Tom's Messages. Inside this folder, you should see a nested Inbox, Sent, Junk, etc. You can leave them in their own folder, or, now that they're in Joe's account, use the drag and drop feature to move them to whatever folders you'd like.
I hope this helps! Let me know if you have any questions.