Quote:
Originally Posted by n5bb
... a typical email message is forwarded through many different servers. If any of those transfers fails, the failure message is sent to the email address in the “FROM” header.
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Actually the report about failure of delivery is sent to the address in the Return-path header, which is normally the same as the address in the From header, but not always. AFAIK an SMTP server adds the Return-path header and puts there the "envelope from" address (the one reported by the sender in the SMTP "From" command).
I tested this by sending myself a message with different "From" header and envelope from (using the "Send a copy" function in Fastmail) and the address in the Return-path wasn't the address in the From header but rather my Fastmail userid
(surprising and disappointing: I am quite sure that in the past the default identity address was used there, and I never want my userid to appear anywhere but in the login screen).