The backend is: EMAIL! :)

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.

See e.g.:

https://delta.chat/en/chatmail

https://providers.delta.chat/

Last time I tried delta chat they didn't have chatmail so this is new to me.

Kinda surprised to find this on privacy policy of the default chatmail instance:

> unconditionally removes messages after 20 days

Didn't see any warning about this in the GUI or the chatmail page.

Does this mean after 20 days the messages on my app will disappear? or just they'll only be available on my local app after that point?

In any case, that's a pretty big red flag that it's not clear.

It is deleted from the server only, it stays on your devices, and is also synced to new device when you add one.

Where did you see this? Or just learned from experience?

FAQ: What happens if I turn on “Delete old messages from server”?

https://delta.chat/en/help#what-happens-if-i-turn-on-delete-...

Cannot find it now, but that's how it was discussed at the time when chatmail server became a thing. It is also how it is configured in chatmail:

https://github.com/chatmail/relay/blob/96a1dbac08441034c5990...

https://github.com/chatmail/relay/blob/96a1dbac08441034c5990...

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.

The FAQ reads:

»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.«

https://delta.chat/en/help#what-happens-if-i-turn-on-delete-...

AFAIK, the downloaded messages are not deleted from your app.

So if you don't use the app for more than 20 days, you'll start to lose your oldest messages (that you haven't downloaded to your device yet).

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.

Are there any problems with the fact that email wasn't created to be instant messaging? Are the messages sometimes delayed?

I have not noticed that problem, the messages are delayed by a few seconds, but not noticeably. Only using chatmail / own postfix though, YMMV.

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”).

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

I think any competent email provider will throttle or block you the first time you send a burst of messages.

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.

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?

They do. All email providers have limits either specified or unspecified. It's usually a few hundred per hour and each recipient uses one quota.

This is why they provide a choice to use their own servers

Then why implement email as a backend if email cannot actually be used as a backend?

That's the tricky part of having email as backbone technology. It's an old and varied ecosystem.

I host my own email server and I am pretty sure chats will be instant with it as well as with their servers.

However, some email providers implement various throttling mechanisms.

If you are OK waiting for message to arrive or be dropped silently, you may continue to use Google and Microsoft email.

The UI was actually taken from Signal in the early days[0].

[0] https://support.delta.chat/t/list-of-all-known-client-projec...

Matrix has smooth chat sync across devices despite E2EE, so both don't exclude each other

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.

It doesn't. I don't know how you can deny it at all, it's core Matrix functionality.

Even WhatsApp has sync by now (even if it's artificially limited and works differently).