Sendmail

Self-hostedOpen sourceCommunication

Message transfer agent (MTA).

Same orbit

Federated microblogging server.

Self-hostedOpen sourceCommunication

50,453 stars

Advanced forum / community solution based on Ruby and JS.

Self-hostedOpen sourceCommunication

48,022 stars

Communications platform that puts data protection first (alternative to Gitter.im and Slack).

Self-hostedOpen sourceCommunication

46,343 stars

Platform for secure collaboration across the entire software development lifecycle, can be integrated with Gitlab (alternative to Slack).

Self-hostedFreeCommunication

39,454 stars