Before CU2 you could only send email in exchange 2013 to an external contact via power shell, Now we can do it via the EAC also.  Below we do it via the EAC first,  scroll down to see how to forward to a contact in Exchange 2013 powershell.

Send Email To External Contact Via GUI / ECP

To start open the EAC and create a new Mail Contact in Exchange 2013 with the correct name and External SMTP email address.

Recipients > Contacts

Once done browse to the mailbox that you would like to forward mail from to the new contact and choose edit, then select Mailbox Features and Mail Flow.

Recipient > Mailboxes > mailbox > Mailbox Features > Mail Flow

Then simply enable Mail Forwarding , browse and select the Mail Contact we created in the earlier step.