Hi Dennis,
First of all I'm glad that Marian had a successful operation and is recovering well. All the best wishes to you and your family from the other side of the Planet.
Regarding mail bouncing, one thing is strange. You say Groups.io relayed messages were bouncing, yet you received an error notice mail from that same Groups.io. When mails bounce, the sender normally receives a mail notice with the exact error code and additional description. However in your case I the sender is a robot (Groups.io), so the only chance for more info is Groups.io's courtesy to provide it to you.
The only scenario I can think of is that your ISP temporarily had Groups.io on a spam list, because somebody on Groups.io violated the terms of use. ISPs are actually clusters of ISPs (the bigger providing services to the smaller), and they usually share some trust infrastructure. That infrastructure consists of certificate authorities (CAs) for S/MIME and other certificates, and also peripheral stuff like spam list filters. The renew of expired certificates can sometimes fail, and contamination of spam lists takes time to clear up.
Such issues normally get noticed and addressed by web administrators (webmasters). If you have time it may make sense to mail them and inquire about the issue.
Best regards,
Nenad Filipovic
