Frank Tegtmeyer writes:

...

The Internet mail message specification (RFC 822) defined a header to control error reports.

That's wrong. Errors go to the envelope sender. There are many, many bad gateways that do it wrong but there number still doesn't make it right. Quoted from RFC822:

: 4.4.4. AUTOMATIC USE OF FROM / SENDER / REPLY-TO : : For systems which automatically generate address lists for : replies to messages, the following recommendations are made: : : o The "Sender" field mailbox should be sent notices of : any problems in transport or delivery of the original : messages. If there is no "Sender" field, then the : "From" field mailbox should be used. : Dieter