Mattermost

Mattermost is a self-hosted workspace for team communication that combines channel and direct messaging, threads, file sharing, voice and video calls, and workflow automation in one web and desktop application. It supports plugins and bots, playbooks for incident and project workflows, integrations with tools such as GitLab and Jira, and identity handling through LDAP or SAML.

Organizations use it as a searchable replacement for scattered email threads.

Hosting it yourself means conversations and attachments never leave your servers, you are not billed per active user, and the PostgreSQL database can be snapshotted and restored on your schedule.