One problem with the whole C2S thing is you can easily end up in the Matrix/XMPP situation where some clients and servers implement some smattering of standards and it just becomes confusing to navigate and use.

This is approximately the situation in the fediverse today, with Mastodon in the role of Gmail. The difference is that email has IMAP and SMTP, client protocols that are not only standardized but universally adopted. Every email server implements them and every email client expects them.

Tuta has entered the chat.