I am getting the following error in my Event Viewer:
This is an SMTP protocol warning log for virtual server ID 1, connection #15. The remote host "18.104.22.168", responded to the SMTP command "mail" with "453 #4.1.8 Domain of sender address <firstname.lastname@example.org> does not exist ". The full command sent was "MAIL FROM:<email@example.com> SIZE=5668 ". This may cause the connection to fail.
I picked this up when I received an email from my ISP saying that my mail server was sending excessive spam. I enabled logging on SMTP and received the above error message. It continually occurs at random intervals - at least once every 10 minutes or so. I believe it is spam as I do not recognize the Mail From address. I have blocked the domain in ISA using the SMTP filter but I still get the message showing up in event viewer.
SMTP relay is disabled on my server.
Please advise me as to what should be my next step in:
1- making sure my server isnt being used to send spam
2- stopping the error from showing up in my event viewer