This tip comes from Daniel Anderson's blog. Thanks for your help!
First, create a new mail contact in Exchange 2007:
- Using the Exchange Management Console, click on Recipient Configuration.
- From the Actions pane on the right, select “New Mail Contact”.
- From the New Mail Contact wizard, select New Contact and click next.
- Either accept the default Organizational Unit for the contact or change it as needed.
- Complete the contact information and then press the Edit button at the end of the External e-mail address field.
- Enter in the SMTP Address of the external email address, click Next and then click Finish.
Now we can forward incoming Exchange 2007 mail to the new contact:
- In the Exchange Management Console go to Recipient Configuration and expand the tree to display the Mailbox branch.
- Right click on the mail box you want to use and select properties.
- Click on the Mail Flow Settings tab, select Delivery Options and then Properties.
- Check the “Forward To” box and then click Browse.
- Locate the Mail Contact that you created and click OK.
- If desired, select the “Deliver message to both forwarding address and mailbox” option.
- Click OK and you’re done.