High “Soft-bounce” rate to Comcast email domain

I am an Xfinity customer and work with a nonprofit that is experiencing an extremely high soft bounce rate to only the Comcast email domain users on our organization's email list.  Comcast soft-bounce rate has been as high as 90% on some emails, but sometimes as low as 1%, with no apparent difference in how emails are sent.  Only the Comcast domain has this problem. Our DMARC and SPF/DKIM records are set up and no other domain returns this issue to us. What can we do to get that resolved? Our Comcast users who signed up for our email list are important to us and are confused about not getting their emails. Thank you!