Zulip is an open-source team chat application for teams that work remotely or across time zones. Every conversation starts with a short topic, so you can follow discussions separately, reply in context and pick them up later. Zulip has desktop and mobile apps, integrations with other tools and an API. You can choose between Zulip Cloud, a hosted service, and self-hosting, for example on Ubuntu or Debian, with Docker or with prebuilt images.
Zulip is made by Kandra Labs, Inc., a US company. The software is released under the Apache 2.0 licence and its source code is on GitHub. According to the project site, self-hosting gives you full control over your data and lets you run Zulip behind your own firewall. For European organisations with strict requirements, Zulip is therefore mainly of interest as a self-hosted system. With Zulip Cloud, the service is operated by Kandra Labs, which lists Amazon Web Services, DigitalOcean and Google as infrastructure subprocessors; you then need to assess location and jurisdiction yourself.