1- Create a Contact in ADUC, add the forwarding SMTP address, e.g. mickey@Gmail.com.
2- Go to the user’s account in ADUC.
3- Under Exchange General tab, click on Delivery Options button.
4- Under Forwarding address, select Forward to and click on Modify button.
5- Select the Contact account you just created and click ok.
6- If you want email to be sent to both, select the option “Deliver messages to both forwarding address and mailbox”.
7- Click Ok twice, you’re done.
Repeat the steps above for as many mailboxes as you wish.