Emails to only Xfinity recipients bounce back

I work at a retirement community. Each week, various employees send out emails to all residents at the community.

Recently, we started getting bounce backs on ONLY the residents that use Xfinity email.

Server at mx1.mxge.comcast.net (2001:558:fd02:243f::2) returned '421 4.4.2 Connection dropped due to SocketError

I can send individual emails to xfinity users, even the same email that was sent out to all residents, and they deliver fine.

We have been sending mail this way for years with no issues, then suddenly started having issues.

Any ideas?