At work, we sometimes got abuse mails (the mails mention FBL: Feedback Loop [0]) which, while they do not contain addresses, contain message-id’s which can then

At work, we sometimes got abuse mails (the mails mention FBL: Feedback Loop [0]) which, while they do not contain addresses, contain message-id’s which can then be linked to the original mail. But looking at the list of addresses we blacklisted because of them, I now realize that those are actually all web.de and gmx.de (crappy German freemailers), so it seems the leaks only happen with them. [0]: https://en.wikipedia.org/wiki/Feedback_loop_(email) https://en.wikipedia.org/wiki/Feedback_loop_(email)