Haven't seen this project before - and I've tried to search for mail servers using document-oriented DBs, although not sure if I've searched specifically for MongoDB. Will definitely take a look.
I guess the project is too new and it is hard to stand out amongst other players that have been around for 20 years by now. Even though not having been around so long has advantages as well, for example it allows skipping non-needed features and adding new stuff like full unicode support – I host андрис@уайлддак.орг on my WildDuck test instance (even though I can only communicate with Gmail using this address, other servers either refuse my messages or do not send to such address).
