help with why I am repeatedly not receiving group emails sent by organizations

I have used comcast.net for 30? years now.  For the last several months I am not getting deliveries of some very important information. I have repeatedly asked the groups to try again and it fails to reach my inbox each time.  One is a county organization that includes maybe 30 people. Another is paperwork coming from a doctor's office- it may be a third party mailing. Why would these be blocked? How do we fix it? (I do have a large amount of saved emails and ones that should be purged but it is a huge undertaking at this point.)