I am just catching up to this topic. I may have an idea about why this is happening. I’ve been an owner of an lsoft email forum for many years. Many of the owners and moderators of these lists starting having trouble with multiple subscribers failing to notice the forum digests were going to their spam boxes. The many isp’s saw this glut of mail from one sender and thought it was spam, blocking that mail off their servers. Thus hundreds of folks with AOL or comcast addresses for instance would not get their mail and think that the moderator was blocking them.
Try asking your subscribers to white list your digest email, even one person accidentally marking your digests as spam on a common isp can cause all mail to be blocked to everyone on the same isp. If you have a way to get error reports about sent mail, check into this to see if some isps are blocking the mail.