The only mailserver before this that stepped away from this methodology as far as I can remember was DBMail, but it suffered from many other issues instead.
Using Blob-store like S3 (with some kind of encryption) combined with a db with metadata makes a lot of sense to me for this application. Alternatively, storing it all in something like MongoDB's ReplicaSet would also be interesting.
Now, if ElasticInbox is the best tool for this is a separate discussion, but this type of architecture sure is compelling.
TL;DR - Looks great for large businesses or maybe as a platform for a startup, but not as a hobbyist project.
