A group of which I am a member posted a message to say that messages sent by the Group were not being delivered.
Similar email delivery problems have manifested themselves on a group I Moderate, and examination of my own email delivery history shows my service provider bouncing emails from Groups.io because...
mx.lb.btinternet.com: 522 email sent from 184.108.40.206 found on industry IP blacklists (Spamhaus/Invaluement/ReturnPath) on 2020/10/25 15:20:41 GMT. To protect our customers, we use leading industry providers of blacklists to ensure only good senders can send email to us. If believe this is a mistake, please contact them directly as there is nothing our Postmaster will be able to do.
I don't know how many mail service proividers may have acted upon this erroneous blacklisting. FWIW this problem seems to have arisen recently, such as this morning UK time. Other than on the group I moderate my Groups.io activity is via the web UI so I have no means of knowing how widespread this problem might be.