I'm messing around with Delta Chat and the concept of it working on top of email is pretty damn cool. It lets you send attachments as well, which is for me 100% required in a messaging app, and it's got apps for all the major operating systems.
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(Sadly looks like I won't get to use it with my friends cause they are hell-bent on using Facebook's Messenger. Sigh.)
So Delta Chat has two modes. You can use to "for chats only" (the default—it'll keep chats separate from mail although chats will briefly pop in the normal mail's inbox before Delta Chat can get 'em) or "for all mail" (there are some pitfalls so use with care, although it's what I use).
In either mode, though, you can start a chat with anyone who has a normal email address. It won't be encrypted unless their mail client supports Autocrypt (and has it turned on—like, k9mail has great autocrypt support but it's off by default) but from then on, Delta Chat will see that thread as a chat and grab the mails from that thread.
Even though I'm on fedi, jabber, libera, oftc, tilde.chat, and slashnet, it's Delta Chat that I use by far the most.
@delta