Gmail Message Unique Identity

Question

What is a unique identity corresponding to a gmail message?

X-GM-MSGID 1462906561521916648

or

Message-ID that you get in headers of a mail

Which of them is permanent? Are they unique for a user and identify a message uniquely for a user or are they unique for a user only and two users with same/different message can have the same messageid?

Thanks

Answer 1

Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups.

I'd go with that, as it's independent of GMail, guaranteed to be unique, well documented, and used since the dawn of time.

Further: Google commented that X-GM-MSGID could theoretically change.