turn on SMTP Authentication for forwarders?

I established a mail forwarder for a new domain and my test message returned "550 Please turn on SMTP Authentication in your mail client. mail-io1-f46.google.com [209.85.166.46]:34530 is not permitted to relay through this server without authentication."  The link provided did not work and I am stymied as to how to turn this on.  Note that I am only using forwarders - no email accounts on the domain email.