When I searched for this I had a very hard time finding a right answer because all the results were SEO blogs advertising their newsletter services (Mailchimp, Convertkit, etc.), which is not the same thing.
So I wrote this overview covering all the options I found. Additional input is welcome.
Even though Listmonk seems like the best free & open source option, there aren't many guides for it and the documentation is quite limited. So [I've been having trouble getting it running on a CentOS server](https://github.com/knadh/listmonk/issues/1004#issuecomment-1...).
It's a shame that it seems to be used by thousands of people but almost no one bothers to create guides or improve the docs.
