My store cannot send emails. When I test in the Control Panel, I get an error similar to "5.7.1 Unable to relay" or "550 5.7.1 Relaying prohibited". What should I do?

This is outside of ProductCart and pertains to your Web hosting provider's SMTP settings. This article well describes the issue.

The solution is to ask your Web host exactly which settings you should use to be able to successfully send messages from a Web application. For example, your Web hosting provider may require SMTP Authentication