My number one product I wish existed is this: A complete email server package that is easy to configure, setup, manage, is secure, and is accepted by other email service providers (gmail, yahoo, etc) out of the box. And with easy I mean as easy as apt-get install or just downloading a binary.
For anyone that has configured email servers, you know it is a headache, this tutorial makes it look easy but its only adressing a a tiny portion of the problem (albiet a important one) - email spoofing. (EDIT: it does mention spam assassin at the end so there's a little bit of info about spam filtering)
Mail-in-a-Box [1] is very well-done, with a clean and modern mail server configuration. However it is not very customisable and requires a dedicated vm.
Can I add "mailing list with a decent UI" to the tick list? Something like discourse's UI but with proper mailing list functionality, like Mailman. I know discourse has fairly theoretical mailing list mode but it's atrociously bad...
Check out Hyperkitty (https://gitlab.com/mailman/hyperkitty)