When you send an email through Gmail, a unique Message ID is added to the email header as per the RFC 822 specification. To know the ID of your message, open the email inside Gmail, go to the 3-dot menu and choose Show Original. The RFC822 Message ID will be displayed in the first line of the email message header, as shown in the screenshot.
The Message ID of a email message is exactly the same for the sender, the recipient, a shared email, and migrated email. That means if the recipient opens a member of the shared label or the header of your email in their mailbox, the Message ID will match that of the message in your Gmail sent folder.
Gmail offers a lesser-known search operator – rfc822msgid– that helps you search and find email messages by their Message ID. This search operant is explained here.
So if the Message ID of your email message is firstname.lastname@example.org, a search query like rfc822msgid:email@example.com will return the exact email message in the search results.
And that’s the trick. This search query will work for the recipient, the email sender, and all shared mailboxes. So if you pass the message ID to the recipient, they can simply use the rfc822msgid operator to locate a specific email from you in their own mailbox. Or, you can simply copy the URL of the Gmail search page and pass them to the recipient. The URL will also work for them since the Message ID is the same for them.
You can also use this search trick to bookmark emails in the browser.