Tim van Dalen

Gmail message ID finder is a small Chrome extension that adds a 'Copy message ID' button to the Gmail message menu. When you click it, the RFC 822 message ID of that message is copied to your clipboard. With this ID, you can find the exact message again later very easily.

I personally use it for keeping track of important emails in our CRM.

Because this ID is a property of the email itself, and not something that this extension provides, it works everywhere. If your colleague got CC'd on an old email you'd like to point them to, you can pass them the message id and it will work for them as well.

It also works great in Inbox and the Inbox & Gmail Android apps.