Being mostly offline is an intrinsic property of mobile clients. They need to save network transfer and battery consumption to avoid spending too much money or constant charging. Whisper protocol, on the other hand, is an online protocol. Messages are available in the Whisper network only for short period of time calculate in seconds.
Whisper Mailserver is a Whisper extension that allows to store messages permanently and deliver them to the clients even though they are already not available in the network and expired.
From the network perspective, Mailserver is just like any other Whisper node. The only different is that it has a capability of archiving messages and delivering them to its peers on-demand.
It is important to notice that Mailserver will only handle requests from its direct peers and exchanged packets between Mailserver and a peer are p2p messages.
In order to store messages, one MUST implement the interface below and MUST register it within a Whisper service. The only known Whisper implementation that allows that is [geth](https://github.com/ethereum/go-ethereum).
Mailserver delivers archieved messages to a peer after receiving a Whisper packet with code `p2pRequestCode`. Messages are delivered asynchronously, i.e. a requester sends a Whisper packet with code `p2pRequestCode` and at some point later, it will start receiving Whisper packets with code `p2pMessageCode`.
How a peer can initialize the request to a Mailserver is up to the implementator. Status peers acting as Mailserver expose two additional JSON-RPC methods: `shhext_requestMessages` and `shh_requestMessagesSync`.
Because all packets exchanged between a Mailserver and a peer are p2p packets, all filters created by a peer from which it expectes to receive archived messages MUST allow processing of direct peer-to-peer messages.
Since a mailserver is delivering expired envelopes and has a direct TCP connection with the recipient, the recipient is vulnerable to DoS attacks from a malicious mailserver node.
