Notice: This is only my reasoning
In the BigTable, Row Key is a email address, a unique identifier and a Sender & Receiver. Column families are meta data. (e.g. from, to, cc, bcc, reply-to, ..., etc)
From: To:
================================================================================================================
edwardyoon@apache.org From:edward@udanax.org <"Edward J. Yoon">
To:chanwit@gmail.com <"Chanwit">
edward@udanax.org From:hama-dev@incubator.apache.org <"hama-dev">
Subject: Content: Address-Book: ...
================================================================================================================
<"Hello, I'm ed"> <"This is a content"> Address-Book:doug@apache.org <"Doug Cutting"> ...
Address-Book:channy@gmail.com <"Channy Yoon">
...
Social Email Graph
Notice: It is also only my reasoning