A couple users got their subscription disabled on a mailing list I admin (accounts-enwiki-l) due to excessive bounces. Turns out, it's been affecting everyone subscribed to the mailing list. Here's a sample of the bounce notification, with email addresses anonymized:
firstname.lastname@example.org SMTP error from remote mail server after end of data: host gmail-smtp-in.l.google.com [184.108.40.206]: 550-5.7.1 Unauthenticated email from yahoo.com is not accepted due to domain's 550-5.7.1 DMARC policy. Please contact administrator of yahoo.com domain if 550-5.7.1 this was a legitimate mail. Please visit 550-5.7.1 https://support.google.com/mail/answer/2451690 to learn about DMARC 550 5.7.1 initiative. r7si4651626qhc.49 - gsmtp email@example.com SMTP error from remote mail server after end of data: host mx2.hotmail.com [220.127.116.11]: 550 5.7.0 (BAY004-MC2F17) Unfortunately, messages from (18.104.22.168) on behalf of (yahoo.com) could not be delivered due to domain owner policy restrictions.
From what I can tell by reading the email that triggered the bounce notification, it's a legitimate email sent normally and just CC'd to the mailing list - no spoofing or anything going on as far as I can determine.