In general, emails have a unique identifier that is called "Message ID" which includes an @ symbol though it is not an email address. This can be seen in the headers of a message, and the sender and recipient both see the same code.
An example from Gmail:
2c5001d9dbf6$db01db30$91059190$@gmail.com
Amazon AWS has a service called SES (Simple Email Service) which is a SMTP server. When Amazon talk about identifying a single unique message, they will quote what looks like a GUID, and it has no @ symbol.
An Amazon example:
010201892707c4f4-2103ffe5-812d-473d-95e8-177e4b8b2528-000000
To turn Amazon's ID into a proper Message ID, you need to add the @ on the end, plus the name of the Amazon SMTP server which was responsible for sending the message. Amazon have a few different ones, named after which part of the world they are in. (Your SES account may only have access to one server, which makes it easier to know which to pick.)
Turning my Amazon example above, into a proper Message ID, requires knowing it came from the West EU server, and then it looks like:
010201892707c4f4-2103ffe5-812d-473d-95e8-177e4b8b2528-000000@eu-west-1.amazonses.com
If you use Gmail, you can search for Message IDs using the filter rfc822msgid to find just one message in your mailbox.
Example usage:
rfc822msgid:010201892707c4f4-2103ffe5-812d-473d-95e8-177e4b8b2528-000000@eu-west-1.amazonses.com