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In eugeni's /var/log/mail.log, we have lines like

May 21 21:52:45 eugeni/eugeni postfix/smtp[13499]: 43ABBE14D1: to=<arma@mit.edu>, relay=mit-edu.mail.protection.outlook.com[104.47.41.36]:25, delay=1.7, delays=0.01/0/1.6/0.13, dsn=5.7.606, status=bounced (host mit-edu.mail.protection.outlook.com[104.47.41.36] said: 550 5.7.606 Access denied, banned sending IP [49.12.57.136]. To request removal from this list please visit https://sender.office.com/ and follow the directions. For more information please go to  http://go.microsoft.com/fwlink/?LinkID=526655 AS(1430) (in reply to RCPT TO command))

I noticed because I was subscribed to one of our mailman lists at my mit.edu address, and I was strangely not getting any of the mails to the list.

I went to the sender.office.com link and clicked, and it emailed me a url which I then clicked on too. It said that the block would be removed "soon". Soon has not occurred yet.

I am thinking that this issue, plus also related ones like #34134 (moved), are caused by the big ramp-up in mailman abuse recently, which is #34256 (moved).