You have some options here, depending on what you want to accomplish. If the user is away temporarily, he/she may want to set up a forwarding rule. If an employee has departed, you could delete the user in the public address book, but add the departed user's email address to the User Name field in someone else's (the supervisor, for instance) person document in the PAB, and the mail will drop into that person's mail. You could create a special mailbox to receive all mail addressed to one or more departed employees. Personally, I like a clean cutoff - just let the mail bounce, and the sender will soon figure out that the employee has gone.