• Signal app and protocol (end-to-end encrypted by default, but centralized server, no federation) - use it daily
• Matrix protocol (end-to-end encrypted, decentralized, federated, user gets to choose app to use) - finds it use, but can be slightly annoying when using multiple devices
• Delta Chat - please see notes below
• Briar - does not rely at all on a central server, so can work even when Internet is down (via wifi, BT or memory cards). Syncs via Tor if Internet is up, which does make the mobile app a battery hog. I have not tested this personally, thanks to Slim for insights.
• Simplex Chat - I haven't tested this

Matrix servers infinitely accumulate user data in the transport layer, namely encrypted messages and attachments as well as cleartext metadata about users and groups. A permanent metadata and room membership record may be ok, or even useful accountability-wise, for government officials but is not suitable for many people threatened by rising authoritarianism where “guilt by association” suffices for prosecution or worse. Operators of servers who can access all the social metadata about its users, and maybe even public messages, are becoming themselves a potential target for hacking or authoritarian repression.
Signal’s key architectural innovations were ephemeral message routing, usable E2EE with metadata-minimization where the transport server only stores messages temporarily and aims to know nothing about anything (except your phone number). Metadata-minimization is why Signal’s founder felt fine to entrust all “Signal Server” activities to Google, Amazon, Cloudflare, Microsoft, our very fine riders of the big tech apocalypse. A big step for cryptography in 2014 but a daring gamble, looking from 2025.
chatmail relays follow the Signal transport model and use SMTP and IMAP only for ephemeral message transport where the server keeps literally nothing except a (random) password and last login time. Turns out, if you hold those ancient protocols right, you can have a metadata-minimizing e-mail transport server with 0,001 EUR of costs per year per address.
https://lobste.rs/s/muk05v/reflections_on_matrix_criticism_over#c_1sijfo

Delta Chat

From the FAQ:

• Images, videos, files, voice messages etc. are handled as usual.
• For performance, images are optimized and sent at a smaller size by default, but you can send it as a “file” to preserve the original.

⚡️ Sign up to secure fast chatmail servers or use classic e-mail servers

• Chatmail relays Github repo - Chatmail relays are designed to be very cheap to run, and are automatically deployed and updated. Official list of relays.
• Besides the recommended usage of chatmail relays, you can use many classic email accounts in chatmail apps. List off email providers known to work with Chatmail.
• Chatmail core Github repo

List of clients is quite impressive, includes Android, iOS, a special "family-geared" Android client, a TUI client, and more.

Push notifications

Instant push notifications available for iOS and Android devices.

They seem to have put careful thought into how to handle notifications, especially on Android. By default, depends on Google FCM Push service or microG.
But you can change this to instead use a "background connection" or a "forced background connection".

Note that classic e-mail servers do not support Push Notifications for Delta Chat users.

Audio or video calls

Not really, at least not yet.

You can invite others to an audio/video chat, but it's essentially a Jitsi meeting (or whatever provider you choose). No ring tone on the other end, so you must have otherwise arranged the "call".

• https://delta.chat/en/help#how-can-i-use-audiovideo-calls-with-delta-chat
• https://support.delta.chat/t/i-made-a-video-call-app-with-15-second-ping
• https://github.com/WofWca/video-call-over-email
• https://github.com/WofWca/webxdc-video-call

webxdc apps

• https://delta.chat/en/help#webxdc
• https://docs.webxdc.org
• https://support.delta.chat/c/webxdc/20

Watch out for insecure copy-cats

As far as I can tell these services, also named ChatMail, have nothing whatsoever to do with Delta Chat.

• https://nextqconnect.com/chatmail
• https://chatmailsecure.com

Delta Chat setup flow

I would like to test using the same profile on multiple devices.
So on device 1 (phone) I downloaded and installed the Delta Chat app from the F-Droid appstore. Opened the app and followed the instructions to create my profile. That was painless.

On my second device (tablet) I downloaded and installed the ArcaneChat app from F-Droid. Opened the app, clicked on "Add second device" on the welcome screen which starts the camera viewfinder. On my already registered device (phone), I opened the app, opened Settings, and clicked "Add Second Device". (Note! Don't make the mistake of clicking the QR code icon next to "Settings", because that will not work for adding secondary device, only for inviting others to a conversation).

ArcaneChat TUI client

I chose to create ~/.local/git/arcanechat, set it up for python using direnv echo "layout python3 > .envrc", then installed the client using pip:

$ pip install -U arcanechat-tui[full]

This installs the arcanechat-tui binary, an alias arcanechat, as well as the deltachat-rpc-server in .direnv/python-3.10/bin/.

Next, how to configure the existing account?

• https://github.com/ArcaneChat/arcanechat-tui/issues/91 - Setup as second device
• https://github.com/ArcaneChat/arcanechat-tui/issues/96 - How to add account as second device?

Create a backup in ArcaneChat Android and transfer it to the computer.

$ arcanechat import <path-to-backup-tar>

Worked flawlessly with a single profile in the first device.