[Email] pulls in the recipients email address
[currentdaynumber] pulls in the current day
[currentmonthnumber] pulls in the current month
[currentyear] pulls in the current year
By combining these tags we can successfully create a dynamic UUID and our key parameter would therefore look something like this:
Custom tags could also be created and added to the default personalization tags above to ensure each key value is unique.
NOTE: the same UUID must always be passed into the key parameter of both the anchor and image tag. If running multiple HTML Ad units within the same email, tags can be rearranged or added to ensure the UUIDs of each placement are unique