Where will the messages be stored? Do you retain them once they’re “received” by a client?

During the alpha phase, messages are stored in XMTP playnet nodes for 30 days, as well as within XMTP clients after retreival.

While many different approaches for long-term message storage are currently being researched, XMTP does not yet currently support message storage at the network level beyond 30 days.

Currently XMTP clients are free to maintain their own retention policies for local storage.