Domains should have to publish in their DNSs the message-id (among any
other thing) through a TXT or A record of any legit mail sent by them.
The TTLs of those records can be adjusted to compensate for queued
mails, etc.
Maybe I'm missing something, but why would you want to do this rather
than a DKIM signature? DKIM validates against the DNS, and protects
the whole message.
R's,
John
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