On Mon, 25 Oct 2021, Ron Garret wrote:
Note that message-ids are not guaranteed to be unique. During my test I found groups of as many as 20 different messages with the same message ID. (Turns out this makes quite a reliable spam signal!)
It's by far not a rare situation: duplicate message-ids happen whenever the sender names more than one local recipient during SMTP. It's a wholly unreliable way to indicates spaminess. However, if a high proportion of those recipients do not exist, ...
I think you may have misunderstood. What you say isn?t wrong, but in the case of multiple local SMTP recipients, all of the duplicate messages will have the same content. What I have found is the same message ID in messages with (very) *different* content (and often sent to the same user). All of that has been spam (and it is hard to imagine any situation in which it would not be).
Ah, that is a different situation. It could happen if the same message tooks different paths to your user e.g. via mailing list processor, but that is less common and would probably break DKIM.
Joseph Tam <jtam.home@gmail.com>