I started my graduation internship two weeks ago. My assignment is to look for a Slack alternative that'll be able to deploy on-premise. A lot of things need to be automated. I already did some research and found some interesting platforms like Mattermost, Zulip, Rocket.chat.
I was wondering if someone has any recommendations that really worth diving into? Also would like your extensive arguments about topics like user interface, integrations/webhooks/bots, security, scalability and so on.