If you try to forward to an external smtp address trough emc , is not possible

solution:

1. Create a contact in the Exchange 2013 “Admin Center”.
2. If you want keep and forward, From Exchange Management Shell: Set-Mailbox -Identity “Allen” -DeliverToMailboxandforward $True -ForwardingSMTPAddress “allen@externaldomain.com” -ForwardingAddress “allen@externaldomain.com”
2a. 2. If you want to forward only, From Exchange Management Shell: Set-Mailbox -Identity “Allen” -DeliverToMailboxandforward $False -ForwardingSMTPAddress “allen@externaldomain.com” -ForwardingAddress “allen@externaldomain.com”

If you go back into the Admin Center and look at the properties of the Mail Flow, Forwarding, you will see your Contact listed.