The mail server, rebuilt for peer-to-peer.

// The server speaks UDX only: give the host that transport.
h, _ := libp2p.New(
    libp2p.NoTransports,
    libp2p.Transport(udxtransport.NewTransport),
    libp2p.ListenAddrStrings("/ip4/0.0.0.0/udp/0/udx"),
    libp2p.Security(noise.ID, noise.New),
    libp2p.Muxer("/yamux/1.0.0", yamux.DefaultTransport),
)

cl := client.New(h, client.Config{
    PreferredServers: []client.ServerPreference{
        {PeerID: serverPeerID, Priority: 1},
    },
})
defer cl.Close()

// Bob is offline. The server holds this until he collects it.
result, err := cl.SendMessage(ctx, bobID, []byte("hello"),
    client.WithEncryption(),
    client.WithExpiry(24 * time.Hour),
)

// Hours later, Bob comes online and collects his mail.
messages, err := cl.RetrieveMessages(ctx,
    client.WithMaxMessages(50),
)

ids := make([]string, 0, len(messages))
for _, m := range messages {
    ids = append(ids, m.MessageID)
}

// Retrieval never removes anything on its own: acknowledge.
ack, err := cl.MarkDelivered(ctx, ids)

ownerID := cl.PeerID()

putResp, err := cl.PutDocument(ctx, ownerID, "config/settings",
    []byte(`{"theme":"dark"}`),
    client.WithContentType("application/json"),
)

// Only succeeds if no other device changed it since.
_, err = cl.PutDocument(ctx, ownerID, "config/settings",
    []byte(`{"theme":"light"}`),
    client.WithIfMatch(putResp.ETag),
)
if errors.Is(err, client.ErrConflict) {
    // fetch, merge, retry
}

cl.CreateMailbox(ctx, "team-updates", wire.MailboxShared)
cl.GrantAccess(ctx, "team-updates", colleaguePeerID, wire.AccessReadWrite)

cl.CreateMailbox(ctx, "announcements", wire.MailboxPublic,
    client.WithMailboxMaxMessages(500),
    client.WithRetentionDays(7),
)

cl.RegisterNotificationHandler(func(n *wire.Notification) {
    fmt.Printf("New message in %s (type: %s)\n",
        n.MailboxPath, n.MailboxType)
})