The first step is to open the Exchange Management Console. Once you open the console you will need to select the recipient Configuration by following the tree on the left panel as follows: Microsoft Exchange>Recipient Configuration.
On the right side pane select appropriate option ‘New Mail Contact’. Enter the appropriate information for the new contact and the remaining ‘name’ and ‘alias’ fields will auto fill from the first, initials, and last name fields. In that same screen press the ‘Edit’ selection to enter in the external mail address. Once you have filled the appropriate fields you will get the pictured below.
Click New and Finish. If you need to modify any of the properties of the contact simply highlight the contact and click properties on the right-hand pane as highlighted below.