Thank you both. Interestingly, the issues I am seeing (at least in terms of subscribers contacting me) are never flagged as bounces in Active Campaign. In all cases "something" has happened at their end, presumably with spam.
I had someone today who swore blind she didn't get it but eventually found it in junk. But she moved it to her Inbox - and it disappeared again. She looked in junk and her inbox but there was no trace. I have no idea what happened there. The previous eMails in our automation were nowhere to be seen, including junk.
Unfortunately, it's these types of issues that are toughest to track down because I'm reliant on an informed subscriber. The subscriber starts with the assumption that it's our problem and so contacts us. My demographic is rather old (on average) and often less tech-savvy.
It's a difficult balance of being there for the customer vs. being clear when it's outside of our control. So I am eager to bring some process to this, which is why I asked.