A guide to generating UUIDs with MailGun
%recipient_email% pulls in the recipient's email address
You could then follow the instructions here to create additional custom variables to use in conjunction with the one above to ensure every email sent out generates a unique key, or you could simply add a static value before or after the "%recipient_email%" tag such as the campaign ID. For example, if we wanted to use the recipient's email plus the campaign ID, our key value would look something like this:
If using the example above, just be sure to update "+123" with the appropriate campaign ID each time a new newsletter is sent out.
NOTE: the same UUID must always be passed into the key parameter of both the anchor and image tag