I’ve been hearing rumors that some *.rr.com domains have been bouncing all mail sent to them. Those domains belong to customers that were moved to Comcast as part of the RoadRunner / Comcast / Adelphia purchase and customer swap. As a courtesy, RoadRunner forwarded mail to comcast for those former RoadRunner customers, but have ceased to do so.
Mail to any address in the following *.rr.com domains will no longer be delivered.
These addresses should be removed from your lists. These users now have Comcast addresses. You cannot just substitute the Comcast domain for the RoadRunner domain as users were required to choose new localparts. That means firstname.lastname@example.org may not be, and probably is not, bobjones@comcast.