Exchange 2007 Unable to Relay

Need an Exchange Expert today.

Running Exchange 2007 (Windows 2008 Enterprise x64), NO EDGE SERVER, SP1.

Problem: Can receive emails from external recipients (Gmail, Yahoo, etc.). Cannot Send emails back out from internal client (Outlook). Receiving the following error:

ExchangeServer.FQDN.local #550 5.7.1 Unable to relay ##

I have been researching this for 3 weeks now. I cannot see a reason why I can receive external emails, but cannot send emails out from internal.

The only thing I have to go off of is the NDR error that isn’t helping me at all.

Firewall ports are open (because I’m receiving mail), IPv6 is off on server and client.

Any help is appreciated. Thanks.