Please try below steps :
Create a MailContact
Create a MailContact using the EMC:
1. Expand Recipient Configuration | Mail Contact
2. In the Action pane, click New Mail Contact
3. To create a new Contact object, leave the default (New Contact) selected | click Next
4. Type First name, Last name
5. Click Edit to add the external email address
6. Click New to complete creation of new MailContact
Forward the Email
Now that we have a MailContact created for the external recipients email address, we can forward mail for the Exchange recipient to the MailContact. To configure mail forwarding using the EMC:
1. Expand Recipient Configuration | Mailbox | select mailbox | properties | Mail Flow Settings tab | Delivery Options
2. Under Forwarding address, select Forward to
3. Click Browse to select the MailContact
4. Optional: If a copy of the message needs to be delivered to both the external recipient and the original recipients mailbox, select the Deliver message to both forwarding address and mailbox
Hi,
The method that Dinesh provided is very comprehensive.
And I found additional useful article:
Create Forwarding Mailboxes for Custom Recipients
http://technet.microsoft.com/en-us/magazine/dd547068.aspx
Hope it is useful
Thanks
Mavis
Its much easier.
Go to EMC
Create a contact
Give the contact an internal email address.
Give the contact an external email address.
Voila