Intermitent Status: 5.1.1 (permanent failure) for some Comcast.net senders to our domain

[Integnia_Admin]

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:

Reporting-MTA: dns; nospam.nonstopdata.net

Final-Recipient: <someuser@mydomain.com>
Action: failed
Status: 5.1.1 (permanent failure)

Some emails from comcast go through even to the same recepients that it fails at times.

Any thoughts, suggestions, advice . Thanks. 

[Comcast_Michael]

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.