There was a fairly active thread on the mailop mailing list about this, as you might expect given its userbase.

Annoyingly, mailop.org list archives are member-only (free to join however).

Mail Archive has the content, though I never much liked their UI. Link to the first message of the most interesting (IMO) thread, though there were others around the same time: https://www.mail-archive.com/mailop@mailop.org/msg26087.html

edited to add: this was a general MS issue and not necessarily senders having spam/dns/domain issues, which I suppose it why it annoyed mailops so greatly! and of course the long tail of the wider internet