For Exchange users:
In Outlook:
Click File
Click Manage Rules & Alerts
Click New Rule
Click “Apply rule on messages I receive” under “Start from a blank rule” near the bottom of Step 1.
Click Next
Click Next again
Click Yes in the dialogue box that pops up to confirm that you want the rule to apply to every message you receive.
Click the checkbox next to “redirect it to people or public group”.
In the box at the bottom labelled Step 2, click “people or public group”.
Click in the box at the bottom next to “To ->”
Enter your external email address.
Click OK.
Click Finish.
Make sure the box next to the newly-created rule is checked and click OK.