In principle this is not a problem in a peer to peer network as long as there are peers. You simply store the message encrypted on enough peers that are in the swarm.
It seems though that they store the message on the sender's device until the peer becomes available:
It seems though that they store the message on the sender's device until the peer becomes available:
http://blog.bittorrent.com/2014/11/21/offline-messages-come-...
Edit: it is fully asynchronous now: http://blog.bittorrent.com/2014/12/22/bleep-now-supports-asy...