There's the Message-ID field, but do e-mail clients (apart from Mutt) let you search for the message with a given Message-ID?

E.g. GMail supports rfc822msgid:<id> searches.

But is it not that the whole thread has same id? I was trying to send myself a text message everytime I get a certain status email. Problem was that all emails had same subject. I was using Google Apps Script. I ended up marking each email processed's arrival date & time when processed; & later skip all whose date time already exists in sheet.

How would you remember a random string if you cannot remember a subject line (or even body text) worded by a human? All email clients allow you to search the subject, body, presence/absence of attachments, from, to, etc. Message-ID, References, In-Reply-To...there are already several headers created and managed (though in a hidden way) by email clients. But the values of these headers are usually long strings that humans cannot easily remember.

Open up an emails from different email threads (preferably one that has been sent later in the reply chain) and see the raw headers. There's a lot of information there. Something in it could be useful to you or for you to devise something better than the status quo.

I use email all the time and have never even remotely encountered a point where I would need something like this. Why wouldn't you simply refer to a subject line? And why would someone say "Did you see email E2YKW3" I can't imagine a single person saying that frankly.

This is what the subject line is for. If you don’t read the existing subject line they write, which distinguishes the various messages from a single person, and gives some indication of the topic, why would you remember some random string of characters that is otherwise meaningless?

That's an interesting thought. I'm working on a desktop IMAP client in my spare time so my burgeoning IMAP knowledge is...getting there.

Would this ID be prominent? Appear on hover?