pump.io : Social server with an ActivityStreams API. It's a stream server that does most of what people really want from a social network.
Riot : Riot allows teams to communicate across a wide range of collaboration apps. If some team members use Riot while others use IRC, Slack or Gitter, Riot ...
SimpleLogin : With email aliases , you can be anonymous online and protect your inbox against spams and phishing. Open-source.
Mail-in-a-Box : Each Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software. It also includes contacts and calen...
Matrix : Matrix is an open standard for decentralised communication, providing simple HTTP APIs and open source reference implementations for securely distribu...
Postal is a complete and fully featured mail server for use by websites & web servers. Think Sendgrid, Mailgun or Postmark but open source and ready for you to run on your own servers.