problem recieving mail from Comcast

I have long-standing postfix / dovecot configuration that's been working fine for years. About 3 or so months ago, I started hearing from friends on Comcast that their emails were bouncing when they try to send to me. I can send to them fine. I've looked at the mail / postfix / dovecot logs and don't see any entries indicating that emails were ever sent at the time-stamp the mail was sent. (copies of the bounce messages were sent to me gmail account).

I've done a bunch of experiments and even set up DKIM signing for outgoing mail, but this doesn't make a lot of sense to me. My friends can send mail to and from other destinations and so can I. I see no evidence that it's being bounced as spam. The only error message is "Delivery to the following recipients failed permanently". There's also a header, X-Authority-Analysis, which I think is a Comcast header but it doesn't provide any useful information

Can someone point me in the right direction on how to debug this problem?

Thanks.