email 550 error for known addresses

Today we've recently started to get emails that wouldn't send and received a permanent failure reply from the Comcast server.  This has occurred when sending emails to addresses we've been emailing for years.  Normally apple mail is used to handle emails.  When attempting to send to those addresses from webmail it gave us a 550 - 550 5.1.1 error.  Though out the day more and more addresses in different domains are unreachable.  We also learned that a couple of people emailing us have gotten a similar message.  Other emails are able to be sent and received just fine.