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 difference 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.
A node which wants to provide `Mailserver` functionality MUST store envelopes from incoming message packets (Whisper packet-code `0x01`). The envelopes can be stored in any format, however they MUST be serialized and deserialized to the Whisper envelope format.
A `Mailserver` SHOULD store envelopes for all topics to be generally useful for any peer, however for specific use cases it MAY store envelopes for a subset of topics.
In order to request historic messages, a node MUST send a packet P2P Request (`0x7e`) to a peer providing `Mailserver` functionality. This packet requires one argument which MUST be a Whisper envelope.
The `Cursor` field SHOULD be filled in if a number of envelopes between `Lower` and `Upper` is greater than `Limit` so that the requester can send another request using the obtained `Cursor` value. What exactly is in the `Cursor` is up to the implementation. The requester SHOULD NOT use a `Cursor` obtained from one `Mailserver` in a request to another `Mailserver` because the format or the result MAY be different.
Historic messages MUST be sent to a peer as a packet with a P2P Message code (`0x7f`) followed by an array of Whisper envelopes. It is incompatible with the original Whisper spec (EIP-627) because it allows only a single envelope, however, an array of envelopes is much more performant. In order to stay compatible with EIP-627, a peer receiving historic message MUST handle both cases.
In order to receive historic messages from a `Mailserver`, a node MUST trust the selected `Mailserver`, that is allowed to send packets with the P2P Message code. By default, the node discards such packets.
For a requester, to know that all messages have been sent by `Mailserver`, it SHOULD handle P2P Request Complete code (`0x7d`). This code is followed by the following parameters:
If `Cursor` is not empty, it means that not all messages were sent due to the set `Limit` in the request. One or more consecutive requests MAY be sent with `Cursor` field filled in order to receive the rest of the 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.
