<https://forums.hak5.org/topic/26861-sendmail-open-relay/#comment-205285>
<http://schema.org/text>
"Smtps on port 553 is encrypted but standard port 25 smtp is not. There are a lot of people who have multiple smtp servers in their settings and if their mail client is unable to connect to their main smtp server they get a choice to use one of the other servers. If they select a standard smtp on port 25 then the redirect would work and the traffic would not be encrypted. "
<https://forums.hak5.org/topic/26861-sendmail-open-relay/>