Mattermost is in the Message Boards category. Mattermost is an open source Slack alternative. It's written in Golang and React and runs as a single Linux binary with MySQL or PostgreSQL. Use the features you like (file sharing, real-time group chat and webhooks—to name a few) and access the source code. See Mattermost vs. Slack
What if you could get a bigger list of Mattermost websites as well as the email addresses?
Click here to download unlimited lists and quickly grow your business using an outreach strategy, you have data for years and years: