I am having this strange issue with some comcast.net senders emailing my servers recepeints and on their end they are getting Status: 5.1.1 (permanent failure). It only happens with Comcast.net senders.
The comcast sender emails someuser@mydomain.com and gets a bounce back email with this:
Thanks for reaching out to us on our Comcast Business Forums, Integnia_Admin.
I'm sorry that some of the Comcast users are having issues getting through. Have you run a blacklist check on your domain? I usually use MXToolbox to check. Click MXToolbox to visit the site and check for domain issues. If you still need assistance from me, send me a private message with your full name, address and the email your senders are having trouble getting through to.