The string is a series of hashes taken from the message. With the new protocol (over port 443) we have the ability to include more hashes for longer message, improving spam effectiveness. Also included is the IP address that sent the message.
This is very useful information when you are submitting TrustedSource udpate requests. We are able to make adjustments based on the message signitures in addition to just the IP address.
Good stuff, thanks for the response.