Delta Chat is a secure instant messenger app with no central account as it transparently uses encrypted mails via your existing mail account so everyone is potentially a user

This is a really interesting concept to have an open source native app running on Android, iOS, MacOS, Windows or Linux which does not require any central registration or account. Just about everyone has an e-mail account onlne so theoretically everyone is reachable with this app unlike Whatsapp only sending to other Whatsapp users.

The messages are encrypted with Autocrypt Level 1 standard and can thus E2E-encrypt messages with other Autocrypt-capable apps. There is a way to even use your existing PGP encryption key, as well as to use it across multiple devices. They answer many potential questions in their FAQ on their website.

It also only shows your actual app contacts so you won't suddenly be flooded with e-mails inside the app.

See The messenger - Delta Chat