It uses SMTP/IMAP to propagate and store individual messages. This means that DeltaChat it will work with your usual email account (it will create an IMAP folder named DeltaChat), but if you install the app and say “Yeah, just let me in!”, it will create a random username for you on one of its own chatmail servers.
It may sound like a bad thing to use email, but it works very, very well. Most people won't even notice.
You obviously must trust that the server runs this configuration, but you can always run your own chatmail, or regular postfix. (If you don't need "federation" with other mail servers, you don't even need port 25 open).
However, you can also configure your app to delete messages from the server sooner.
»By default, Delta Chat stores all messages locally on your device. If you e.g. want to save storage space at your mail provider, you can configure Delta Chat to delete old already-received messages on the server automatically. They still remain on your device until you delete them there, too.«
Delta chat uses email. So you can chat with anyone that has an email address. If they happen to use the app too it'll feel more like a chat for them as well.
Well, for a starter you will be limited to what email servers provide, which doesn't include video conferences, which most instant messaging solutions include these days
Not in my experience. It feels like chatting on Signal or any other instant messenger, except there are no typing indicators, especially if you use DeltaChat's chatmail server (the default if you just say “let me in”).
Wait, is that true over regular email too? Say I have a back and forth one liner emails every few minutes? It's competent for the provider to throttle or block these?
I haven't run into that problem, but yes, it's possible.
One problem I did run into was “allowed number of outgoing emails”. If you use groups in DeltaChat, even a small grouop of say 10 members will incur a lot of outgoing messages. The provider I originally used has a limit of 200 emails per day, so that was a showstopper.
If you use DeltaChat's chatmail server (which will happen per default if you don't provide an email account of your own), this will not be a problem.
It does, and I did try it for a couple of years — I even ran my own server. None of my close friends cared to try it, and I preferred XMPP myself, so I took down the server and delete the app.
What is the backend for it? It's hard for me to find on their website. If it's also Telegram, than what's the point?
I would also like to point out that Telegram has very smooth chats sync across devices because those are NOT end2end encrypted by default.