Comment by ttul

Except it's not an open relay. MailChannels would not exist on the internet if it didn't aggressively control spam and phishing. The DEFCON talk did not prove the existence of a gaping hole; it showed something that has existed since the dawn of internet email: the sender domain cannot be adequately authenticated unless you use message signatures such as S/MIME and DKIM. And even with DKIM, DKIM replay attacks allow widespread abuse.