Zulip. It's Open Source, you can either pay to have it hosted or host it yourself, it has mobile apps and a web interface, it has a good API for integrations, and the conversation model feels great.
Zulip is an excellent user experience, and when deployed self-hosted is also quite secure. The only thing it doesn't have is federation, which would involve rethinking security.
I've found it very effective after deploying a local version for our team.
One issue: Some users get confused over streams and topics, often posting anything in whatever was the last topic they accessed rather than going to the correct topic or starting a new one. These same users also say they can't find anything via search.
> One issue: Some users get confused over streams and topics, often posting anything in whatever was the last topic they accessed rather than going to the correct topic or starting a new one
This has been a problem ever since email: many people reply to the last mail they have from a list, sometimes changing the subject, and then get surprised when the mail is threaded under the mail they replied to.
I've seen this occasionally on Zulip instances, but a reminder and a suggestion that the topics are like email thread subjects or forum thread subjects tends to help.
Indeed, it's very much the same thing.
I have brought up the similarity to email, but it's been of limited help. I'm hoping that moving posts between topics will become possible at some point, which will allow me to clean things up.
It's already possible to move posts between topics. In the menu for a post, choose to edit the topic, and once you start changing it there's an option to move just the post or move later posts in the same topic.
Zulip's UI is pretty laggy in my experience (only used it on a big public server), can't seem to remember what I have already read sometimes and seems to only support threaded conversations (called topics by Zulip).
I haven't hosted it, so I don't know how much work is self-hosting it. Still seems like a better idea than sending all of your company discussions to e.g. Microsoft, especially when you're not from the US and can be the target of industrial espionage by the US government.
Can you share what server this was (here or in chat.zulip.org)? Essentially every bit of feedback we get on performance is that Zulip is much snappier than Slack.
Given that you mention a big public server, it's likely caused by an issue specifically related to that server's scale/usage patterns, and I'd like to make sure we investigate it.