SMTP and the infamous 550 relaying denied

The plan was simple. Create a new email account in Outlook that would connect to to send the message with SMTP. The first couple of tests worked like a charm. Then I sent a reply to a person on and got the:

“550 Relaying Denied. Not A Local Domain: If you are a customer, download messages via POP, and try again.”

Further research at Interland was not encouraging. I tried a couple of things but finally gave up and sent it via Yahoo.