Almost any NoSQL would fit your usecase. For example, a mailbox matches the MongoDb Collection model well.. it's a file on a disk (so there is no outer index o

Almost any NoSQL would fit your usecase. For example, a mailbox matches the MongoDb Collection model well.. it's a file on a disk (so there is no outer index on username needed - filesystem will do it) and you can specify which inner index(es) you would like, probably time. Storing auto-assigns internal 128bit uuids that you can use to refer to a specific message. If you're allergic to mongo you can do this in pretty much any document store. Hell you can do it in a simple SQL table but you will have to deal with outer index (which may or may not be a problem for you).