Often overlooked from our "but e-mail!" skeptics: Any sufficiently advanced P2P messenger will eventually re-invent a custom, partial form of e-mail ... because users want to communicate when their apps are offline or not foregrounded and active at the same time. See https://briarproject.org/download-briar-mailbox/ for a recent example.
Delta Chat goes the reverse route by providing a secure and interoperable e-mail based messaging experience and then adds P2P tech like https://webxdc.org on top.