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 assuming nobody's doing any funny business. I know that with spam, all the rules go out the window...)
According to RFC2822 - Internet Message Format, the short answer is that the "Message ID should be unique for each instance of the message"; however, the MESSAGE-ID field is considered optional and how the MESSAGE-ID field is created is up to the server. Quoted below: