1. msmtp is an SMTP client.

  2. Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.

  3. Blat is a small, efficent SMTP command line mailer for Windows.

  4. AtMail is an open source webmail client written in PHP.

  5. Free utility to send emails from the command line. No Installation. All contained in one file.

  6. CMail is a command line mailer, originally developed for testing mail server software, but is...

  7. qmail is the name of an advanced level of mail transfer agent for the UNIX based operating systems.

  8. Open source SMTP server written in Node.js

  9. MailHog is an email testing tool inspired by MailCatcher, but easier to install.

  10. Zimbra Collaboration is an open, secure collaboration tool for email, calendaring, tasks and more.

  11. A fully fledged, free email server solution, an open source project (GPL v2).

  12. MailerQ is a high performance Mail Transfer Agent (MTA) designed for delivering large volume email messages at very high speeds.