Should I go for document-based or graph database? (MongoDB vs Neo4j) [closed]
Maybe this blog post will help steer your decision: http://soloso.blogspot.com/2011/07/getting-enron-mail-database-into.html
Take note of the very bottom addendum area -- where @rit expanded on the original code and hosts a dump of the corpus on S3.
That ought to give you quite a bit to experiment with in the MongoDB environment.
Bryan