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

for bulk mailing, it's better to be unique for each mail, the message id may be the only information your get back from spam notifications or bounces so it should be unique for each message.