If david@gmail.com then sends an email to onion2k@myemailprovider.com Google know that david@ has a second order link to alice@, bob@ and charlie@ I'm not sure

If david@gmail.com then sends an email to onion2k@myemailprovider.com Google know that david@ has a second order link to alice@, bob@ and charlie@ I'm not sure at all that it is clear what the nature of that link is. I have no insight into the gmail's email relationship modeling but I suspect that it doesn't automatically build a relationship between david@gmail.com and other people that onion2k contacted. For one thing, it would be impractical to do this across all possible relationships -- there would be polynomial growth of the adjacency matrix with no information gain to justify it. Ask yourself, what does it mean that david contacted onion2k and onion2k, at some point in time, contacted alice, bob, and charlie? Unless you already know something nontrivial about these people, you assume they are independent. I guess my point is that data analysis can be very costly; it is reason that humans have a hard time dealing with meaningful relationships spanning more than about 150 people[1]. Even a company with Google's resources would be overwhelmed if it had to store and take into account a large number of mostly (I mean, vastly mostly) meaningless relationships. At the least, it takes away resources from the relationship graphs that do matter and which are actionable, such as the graphs of spammers. [1] http://en.wikipedia.org/wiki/Dunbar%27s_number http://en.wikipedia.org/wiki/Dunbar%27s_number