Is the “Message ID” Email Header unique for each recipient?

Question

How unique is the Message ID header of an email? If I address an email to two people, will the both have the same Message ID? Or will they be different?

(This is assum

Answer 1

Typically, all recipients will receive exactly the same message, both body and headers, with the exception of headers that are added en-route (eg: "Received" headers). The Message-ID header is generated by the sender, so all recipients should see the same Message-ID.