Delta Chat

Delta Chat is a messenger that uses email as its transport. Messages go out over SMTP and arrive over IMAP, so contacts are email addresses rather than phone numbers, and the person on the other end can be reached even if they never install the program — their side simply sees mail. Any mail account can be used by entering its credentials, and the project also runs lightweight servers set up specifically for this kind of traffic.

What is presented is a chat client rather than a mail client: messages are grouped into conversations, quoted replies and reactions behave as expected, and the usual clutter of headers and subject lines stays out of the way. End-to-end encryption uses Autocrypt, which exchanges public keys through message headers and engages automatically once both sides support it; a message to a correspondent whose key is unknown goes out unencrypted.

Conversations can also run small self-contained applications shared between the participants — editors, polls, games — which arrive in the chat as a file, run locally, and pass their state around as further messages.