If this is for legal reasons to monitor what mail is being sent to the first user and you have permission from upper management and HR, then you can go into ADUC, open the account, go to the exchange general tab, click the delivery options and enter a name in the “forward to” field. Also check “deliver message to both forwarding address and mailbox”. That takes care of mail being sent “to” the user, but I do not have a solution for forwarding mail sent “from” the user. You can grant the second user mailbox permissions on the Exchange Advanced tab to the first users mailbox and if they do not make any changes they can review the mail in the first user’s mailbox without their knowledge.
Yes, this is a legal reaons dictated by my boss, don’t worry. I had known “the deliver message to both forwarding address and mailbox”. That takes care of mail being sent “to” the user” and its only take care of viewing of mail coming to this users. We need also to track outgoing mails from this users (internal sending out or external sending out). I used your method of Exchange Advanced tab to the first users mailbox. But it only seen mail coming from, NO going out mail is being monitor or being seen.