title: iptables port redirection
body: <p>I have following problem:</p>
  
  <p>CRM system running on Linux computer is sending emails via sendmail.</p>
  
  <p>I would like to change destination port from 25 to 587 using iptables but without modifing 
  sendmail (not possible  due to crm configuration).</p>
  
  <p>I tried to use iptables but mine tries of redirecting packet on output chain to port 587 were unsuccessfull. </p>