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.
Mailtrain : Mailtrain is a self hosted newsletter application built on Node.js (v5+) and MySQL (v5.5+ or MariaDB). Mailtrain supports subscriber list management, ...
Novu : The open-source notifications infrastructure. A fully-featured microservice to send SMS, E-mail, Slack and Push notifications. Embeddable notification...
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...
WireMock : Free and Open Source tool for building mock APIs. Create stable development environments, isolate yourself from flakey 3rd parties and simulate APIs t...
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.