The message ID in Gmail is a unique identifier for each individual email, regardless of its content, while the thread ID identifies a whole conversation or series of related emails. If likened to a chain, the thread ID represents the entire chain, whereas each link has its own distinct message ID.
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How to get message ID in Gmail
In Gmail, every email has a unique “Message ID” used for internal tracking. To find it, open the desired email, click on the three vertical dots at the top-right, and select “Show original”. The Message ID will appear in the headers, typically inside < > brackets.