title: the server rejected one or more recipient addresses. the server response was 550 5.7.1 unable to relay
body: <p>I am sending mails using our company SMTP server. I get the problem when I am sending mail to other mails (out side of the company domain), returning the error:</p>
  
  <blockquote>
    <p>"The server rejected one or more recipient addresses. The server
    response was 550 5.7.1 unable to relay".</p>
  </blockquote>
  
  <p>If the mail is within the company, then there is no error and mail sending successfully. My web application hosted in IIS. </p>