That's a spam technique that started being heavily used just a couple of days ago.

If you open up the complaint email to display the hidden message routing history, you will see a reference to the email address of your group as the address that email systems are supposed to use to complain about that email.

Google explains how to display the hidden message routing history -- whether you are using Gmail, Yahoo Mail, Apple Mail, or Outlook, etc.

Ignore those messages. (Don't click on any links in those messages -- may send you to a poisoned website that will attack your system.)

--cg