I think Mark is right. The problem will not diminish, and could get worse, if members are not forced to adjust their spam filters to allow for emails from the groups. It's harsh medicine, but as long as spam filters remain as unsophisticated as they are now, email will remain vulnerable to spamming. Spam filters could be much more sophisticated than they are now. Self-learning systems are not quite baked, IMHO, but they are getting better every day and they are well suited to discerning spam filters.
The danger is that email may decline as a common communication method. In software development, for example, there has been a mass movement away from using email for open group communication because spam is too much of a bother. This is unfortunate because the alternatives, mostly messaging systems like Slack, are good for conversation, but not so good for the kind of interaction that occurs in many groups.io groups in which the exchange is of short essays rather than conversation.
Why isn't Oath: paying attention to the chaos in Yahoo Groups? Perhaps because they perceive that email groups are dying. Spam issues are one cause for decline. I think Mark is right to fight it with strong medicine.
Best, Marv
And Happy Easter!
Marvin Waschke