On 2024/08/31 10:14, Christian Schulte wrote:
> 
> "In all cases, it is the meaning that the *sender* of the message wishes
> to convey (i.e., whether this is the same message or a different
> message) that determines whether or not the "Message-ID:" field changes,
> not any particular syntactic difference that appears (or does not
> appear) in the message."

If the sending MUA doesn't include Message-ID then presumably it's not
trying to convey any information about that.

> I then mentioned that I think OpenSMTPD gets it right by not blindly
> adding Message-ID headers because it is not the sender and thus cannot
> decide about the identity of a message which made you question OpenSMTPD
> to get it right.

It is standard in most MTAs (including opensmtpd) to add missing
Message-ID at least to emails which they consider as locally originated.
(Doing this for mail from external origins risks DKIM failure with some
header signing setups).

This was broken in opensmtpd on little-endian machines from 2018-2023
for mail received on 587, and if I read the code correctly it's still
not done for mail received on 465. Though that shouldn't affect non-SMTP
mail submission as done by mail(1).