Reference.Email gives your emails a URL that all recipients can use to find that specific message.
This extension adds a button in your Gmail messages' context menu to generate a search URL unique to this message and copy it to your clipboard. You can paste this link anywhere and give other recipients a breadcrumb to reference that specific email by. How? Using the "rfc822msgid" that each email has in its metadata. The Reference.Email link auto-searches for this value and surfaces the one message that matches. (Note: This is Gmail specific, right now.)