Dear customer:
Firstly, you should enable auto-forwarding on global Settings, you can refer to the following steps:
a. Start System Manager.
b. Double-click Global Settings, and then click Internet Message Formats.
Depending on your configuration, there may be one or more Internet Message Format objects.
c. In the detail pane, right-click the Default object, and then click Properties on the shortcut menu.
d. Click the Advanced tab, and then click to select the Allow Automatic Forward check box.
e. Click OK.
Secondly, create contact for external user, follow these steps:
a. Click Start, point to Programs, point to Microsoft Exchange, and then click Active Directory Users and Computers.
b. Create a contact with an e-mail address that you want to forward mail to. For more information about how to create contacts, click Help Topics on the Help menu, and then type create a mail-enabled contact.
Thirdly, create forward rule in OWA, and check the effect.
Hope it helps. If anything is unclear, please feel free to let me know.
Rock Wang - MSFT
