As title, I can't find any flag about enabling the "mailserver" when starting whisper node.
However, I found a Diagnostic Tool called "wnode"：https://github.com/ethereum/go-ethereum/wiki/Diagnostic-Tool-wnode
This tool can be used to start a normal whisper node and also a mailserver node.
I traced the implementation of mailserver: https://github.com/ethereum/go-ethereum/blob/master/cmd/wnode/main.go
And tried to modify the "Init" way of the mailserver from "symmetric" to "asymmetric". It works well on my local, but I can't find any way to connect it to the whisper node start by "geth" even tried "admin.addPeer("mail server node")" in console and "shh_markTrustedPeer".
My Goal is to retrieve the historic message from mail server by using the RPC API like shh_XXXX.
Does anyone successfully enable the mailserver on whisper node started by geth? Or is there any api for calling RegisterServer(...) in whisper: https://github.com/ethereum/go-ethereum/blob/master/whisper/whisperv6/whisper.go#L211