A shell script that turns a brand-new VPS or server into a mail server with a single command. Sets up webmail, IMAP, SMTP, a contacts application, a calendar, and even a web-based control panel for the whole shebang. Includes spam filtering, greylisting, backing up, and TLS certs for everything from Let's Encrypt. Even does virtual domains. No configuration during setup, and you can't really tweak the settings once it's up and running.