Thanks! Make sure to test it.
Yes, encrypted messages are conveyed in custom_json ops, requiring only Posting Authority signing.
Yes, I'm aware PeakD offers this Sting messaging. However, I don't think my Hive-Mail is a competitor to that. This Sting on PeakD would compete with the likes of Telegram, Whatsapp, Signal, etc. I built Hive-Mail as an improved version of the existing Hive onchain encrypted messages. Simply as that.
Also worth mentioning, paramount in my design was that the only online infra required for the use of Hive-Mail is the very same to any other Hive service: Hive RPCs only. Everything is done locally, including all encryption/decryption. The user can be absolutely sure of what's happening, because the scripts are on his own local machine. And the only way (e.g. for a government) to censor/stifle Hive-Mail would require having the Hive blockchain as a whole censored, i.e. all its RPCs blocked.
This relatively long answer to explain that security and censorship-resistance, not convenience or even efficiency, was the motivation behind the new protocol. It is not meant as a substitute for existing communication protocols. If anything, it could be used to improve the existing onchain encrypted messages.