I am the administrator for a system using shared hosting to host email services, and recently experiencing 5.0.0 failures with the error message below sending to comcast accounts:
all hosts for 'comcast.net' have been failing for a long time (and retry time not reached)
I suspect an IP block, but would like to confirm.
Emails from this system tested successfully to reach other domains.
I am an email administrator of another entirely separate system that DOES receive emails from this shared hosting system successfully. I can provide full headers of the succesful emails, if you need.
Note: I am also an xfinity customer, personally, and emails from this system fail to reach my own comcast.net mailbox... so I can test as needed to determine if the issue has been resolved.