Comcast has been returning messages like this for some of our IPs:
"554 resimta-po-03v.sys.comcast.net resimta-po-03v.sys.comcast.net 220.127.116.11 found on one or more DNSBLs"
Despite finding these IPs to be quite clean in our tests, Comcast appears to subscribe to some obscure RBL that we do not know to check (or they use their own internal RBL). Our relays were not configured to retry these emails from different IPs, and were instead bouncing them back to senders. We've added mitigation to our relays that should correct this. After we're satisfied that the change works in all cases, we will submit a PR to the open source project that we utilize for our relays.