SMTP 550 No such user here

This was just discovered with one SMTP server: the error message: RCPT TO: <somebody@somewhere.com> 550 <somebody@somewhere.com> No such user here Was caused by forgetting to authenticate with the SMTP server.  The SMTP server requires authentication in order to relay email to another domain (in other words, it’s protected against being an open-relay).  The error message from this particular SMTP server […]