Hi jwriter, welcome back!
I highlighted the error code from the bounce message that I believe is most important.
That error leads me to these possibilities:
- Most likely - A Comcast customer recently used their smtp relays to spam mac.com accounts -> to prevent abuse the mac.com mail servers are now dropping connections from Comcast.
- Less likely - The volume of legitimate email being sent by Comcast users to mac.com has exceeded mac.com's incoming email limits.
Either way, it should be a temporary situation that will resolve itself.
If this is an important issue to you the next best step is to open an abuse ticket with Comcast technical support.