1. Do you want to permanently delete the old user and mailbox and just have all future mail sent to a different user.
2. Or, would you like to keep the old users mailbox in tact for now, and forward a copy of each mail sent to an additional person? There are 2 ways to do it, which makes more sense?
To do 1 - Delete users account from AD and Exchange then add an alias to the users email who you want to have the mail sent to. Open up EMC - go to recipient configuration, Mailbox and double click the person who will have the alias. Under the email address's tab, sleect add and type in the old users email address.
To do 2. - Open up EMC - go to recipient configuration, Mailbox and double click the person who letf the companies account. Click Mail Flow Settings tab, Highlight deliver Options then hit properties. In the forwarding address put a check and type in where you want it forwarded then whether or not you want to deliver to both the new and original mailbox