Post Reply Share Subscribe RSS Back To Top Similar Topics Reading This Topic Powered by InstantForum 2014-2 Final asked 5 years ago viewed 6425 times active 5 years ago Related 0Sending mails from Unix to Outlook or other Email account3Sendmail returns error code 550 5.1.1, “User unknown”0How to use Answer After upgrading to firmware 1.6 there is a new option Helo Domain Check at SMTP > Configuration > Receiving SMTP > Settings that is enabled per default. This quick tutorial will help you get started with key features to help you find the answers you need.