bitmessage do not store your messages for years, xdn store it in blockchain, and only recipient can decrypt it (even sender can`t). So you can send your message today and recipient can read it after a years.
There are dozens of use cases for XDN messaging system.

One funny case:
Imagine, you export your private keys in paper format (will be enabled next week). Now you can just send your private keys via XDN network to another person with encrypted message. Recipient just import you keys and can (1) use your funds (2) read your messages history.
XDN technology offers safe keys sending via transaction, that is a crazy stuff

XDN messages features:

- Anonymous - you can`t say who is sender or who is receiver, you even can`t say for sure is there any message in transaction.
- Encrypted with strongest cryptography - only receiver can decrypt his message with his private key. (later we will implement tracking key cryptography for services, like Twitter)
- Attached to transaction, but can be send without money.
- Stored in blockchain, so you can read your message any time. Messages stored in decentralized and encrypted database. 
- Processed by decentralized p2p network.

In fact, DarkNote is the only anonymous encrypted untraceable messenger and cryptocurrency.

Also private private messaging will boost XDN economy. In near time we will enable pool mining via Client.

So user can open DarkNote client application, press start mining button, receive some XDN and send his encrypted message.

Also we are thinking about file transfers in torrent like network aside of XDN network. We study that case.

Stay tuned.