SimpleLogin : With email aliases , you can be anonymous online and protect your inbox against spams and phishing. Open-source.
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.
WireMock : Free and Open Source tool for building mock APIs. Create stable development environments, isolate yourself from flakey 3rd parties and simulate APIs t...
Mailtrain : Mailtrain is a self hosted newsletter application built on Node.js (v5+) and MySQL (v5.5+ or MariaDB). Mailtrain supports subscriber list management, ...
Appwrite : Appwrite is an end-to-end backend server for Web, Mobile, Native, or Backend apps packaged as a set of Docker microservices. Appwrite abstracts the co...
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.