A question re Messages from Jonesy bouncing

Has anyone got time to explain or point to something simple that explains it? _All_ our emails are forwarded on, so there is a problem with the two domains. Why does one get bounced, a very few put into the spam folder, and most get through? What is not being done in most cases? Are most of our headers not using DKIM so that it doesn't matter if it doesn't match? I tried the link offered by Jonesy, but I'm none the wiser. I just don't "get" what is going on.

Nor why, if the problem is between Jonesy and the list software, it should unsubscribe others of us.

Would our not using Gmail help? Lisi