It appears that last few fields in the returned header are responsible for the problems. Julf's mail also indicates why cypherpunks has had a couple of duplicate posts recently. The offending headers are "Return-Receipt-To" and "Errors-To". The "Return-Receipt-To" field is triggering a reaction in some other mailers to bounce back acknowledgement of the mail. Now cypherpunks@toad.com was in the "To" list, and it appears that acknowldegement mail was sent out to cypherpunks again. All this time the "Received" fields are increasing. When there are too many of them--the number is mailer dependent, but is typically 17-20, some mailer along the chain bounces the message. It sees the "Errors-To" line and sends back the bounce to penet. My guess is that a significant fraction of the cypherpunks list is sending anon.penet.fi back one message each per "Return-Receipt-To". Not all that many mailers honor return receipts, but all mailers bounce mail with too many Received fields. Hence the first return receipts sent didn't generate nearly so many errors as all the bounces from the second time the message went out to the list. How we solve this? Well, let's list the mailers involved in the particular message you sent. The first one was the anonymous remailer at caltech. The message from there was directed to cypherpunks, so that's toad.com. From there it travelled through uunet (toad.com's mail gateway for a large amount of traffic) to somewhere in the gza/aktis/ov group of machines. Somewhere in there the return receipt was generated; note the "Return-Path: <jik@security.ov.com>" field. This mailer generated a message back to cypherpunks (toad.com) again. One copy of this went to a machine in uci.edu, which bounced it to penet. I'd say that the mailer which generated the return-receipt back to cypherpunks (assuming that happened) is the most proximate cause. Cypherpunks was in the To: field, not the From: field, and even though your standard reply might go to both parties (assuming the To: field is larger than just you), a return receipt should only be propagated to the original sender. toad.com is a secondary cause, since the Return-Receipt-To: field should probably not be propagated out to a mailing list, but rather acknowledged or discarded before mailing list expansion. Also, since toad.com is not running reasonable mailing list software (which we don't have), it's not detecting duplicate messages sent back to the list and discarding them. Eric