Xfinity soft bouncing some emails from one specific sender.

I receive regular emails from a nonprofit that sends their emails from a Google for Nonprofits email address via MailChimp and have received them for years with no issues. I am also involved at the nonprofit in sending out those emails. Although nothing has changed with the nonprofit sender's email address or their use of MailChimp to send out mass emails, as a recipient, I have recently not received some of those emails because Xfinity has soft-bounced them (They are rejected by Xfinity; they do not come to my Inbox or Spam folder). In looking at the nonprofit's MailChimp report for those emails that I have not received, about 30-35 other people also did not receive those same emails as a result of an ISP soft bounce, all of whom also have comcast.net email addresses. The emails are successfully sent to everyone who uses other ISPs. Additionally, there are some people with Comcast email addresses who do not have this problem. I am stumped as to how to pursue this. Everything points to this being an issue of Comcast rejecting these emails for some Comcast users, although so far it only seems to happen when the nonprofit sends out more than one email to us per week. Two Xfinity agents I spoke with were unable to find a problem though.