Mark wrote this:
"Does he forward group messages to other people? If so, they could be clicking on the Unsubscribe link in the message footers."
There is an Unsubscribe link in the message footers, online? I don't see any.
AFAIK, these things aren't happening via email. If someone clicked on the link in an email message, wouldn't it show that he unsubscribed via email? All his unsubscriptions have been "via web". (In fact, that's half the problem; he has had to use the web because Groups.io stopped sending him group messages by email.)
re: bouncing first messages:
"Sounds like a very user hostile way of doing spam prevention."
I agree. But I'm no expert.
I don't know if they bounce back to the sender, or only to the sender's server which retries the message.
Regardless, I THINK it's unrelated to why he keeps being unsubscribed. Even if Groups.io dumps him because of bounced messages, that should show up in the Activity log, and anyway he is getting dumped even when there were no messages being sent.
I just tried to see what happens if I click the red "leave group" button on the Subscription page. I get a pop-up window with the choice to stay but change my message delivery to Daily Summary or Special Notices only, or to actually leave. Could it be that somehow his web browser is automatically clicking those choices, and activating all of them, first changing his delivery mode, then leaving the group? Sounds far-fetched, but it seems to fit the outcome.
Also I wonder if he reads "Leave group" as meaning, "Now I'm done here so I can move on to my next group," and doesn't realize that it means to unsubscribe. This could be a language issue, although his use of English seems very good.