Here's the step-by-step procedure to forward your internal emails (user mailbox) to their external email address.

First create the contact information.

• In Exchange Management Console
• Click on Recipient Configuration
• Select Mail Contact, right-click on Mail Contact, select New Mail Contact.
• In New Mail Contact, click Next to continue.
• In Contact Information, type the user First name, Last name, Alias, then click Edit.
• In SMTP Address, type the external email address for ex. johnsmith@king.net OK
• Click Next.
• Click New.
• Completion notes should have 1 succeeded, 0 failed.
• Click Finish.

Now, let's use this new contact information to receive messages.

• In Recipient Configuration
• Click on Mailbox, select the user Mailbox to manage. For example, select Jane Santos mailbox.
• Go to Jane Santos mailbox properties.
• In Jane Santos Properties, click on Mail Flow Setting
• Select Delivery Option, then Properties
• In Forwarding Address, put check on Forward to:
• Click Browse and select mail contact.
• Click OK

That's all.