Hi All,
I would just like to try to clarify this topic for future readers. In order to be in compliance with US anti spam laws we require all emails sent to 500 recipients or more to include some type of unsubscribe link before they can be sent out. As a result, if you are testing the email to less than 500 you will not receive the error until you go to send to your marketing list larger the 500 recipients. If you would like to avoid encountering this error all together, as a best practice, we suggest always including an unsubscribe link.
Here is our help article about the Unsubscribes and the CAN-SPAM Act.