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We often get emails blocked that have confusing sender addresses. This is always the scenario: a message marked as spam says this in the actual headers: "From: "Bob Jones" [email protected]". But below the section with headers, MailWatch reports the From address as "From: bounces+234567-91eb-recipient address@sender domain". What does this mean? Why does a seemingly normal email coming from a normal address show up as being from bounces+123456-blah-blah?

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The From address can be set to pretty much anything you want it to be. It's arbitrary text.

Other headers, such as Envelope-From, Reply-To, Return-Path, Sender, and Errors-To may provide more accurate information about who sent the mail, especially - as appears to be your example - in the case of a mailing list.

Mailing list software will often leave the original sender's From address but insert Reply-To to make replies go to the list by default. However, in your case, you've indicated (via comment) that MailWatch was paying attention to Envelope-From. That header was likely injected by your mail server; it noted the SMTP "MAIL FROM" value that way. The mailing list sends mail with that "bounce" address as the SMTP "MAIL FROM" so that any bounces will go back and be handled by the mailing list manager, but normal user agent replies to the mail will use the (From, Reply-To) headers to send to (the list, or the original poster, depending on list policy).

The specific weird in the address (234567-91eb-recipient address) allows the mailing list to determine which address it sent mail to that caused the bounce. After a few bounces, it will either pause email to that address or remove the address from the list.

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  • FYI, in the full headers of these messages, the only attribute where the "bounces+123455" address shows up is "envelope-from". 'Reply-to' and 'from' show a couple of different things that look like normal addresses.
    – WakeDemons3
    Commented Apr 5, 2019 at 17:12
  • @WakeDemons3 thanks, I will add that to the list of headers that can be cribbed from and update the explanation (your mail server, not the mail list, would have put that in there).
    – gowenfawr
    Commented Apr 5, 2019 at 17:15

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