adam42 there's no singular "best" solution. I believe two different solutions are offered above. The Subscribed extension that allows users to receive notification of new discussions if they choose to do so, and the Mailing extension which allows sending emails to some groups or everyone without an option for the user to opt out.
There isn't an extension for proper newsletters at this time. If you need the ability for users to subscribe and unsubscribe, I recommend using an external newsletter service.
I will probably publish an extension that allows integrating Flarum with Infomaniak Newsletters in the near future, which adds a newsletter option in the user settings which uses the remote Infomaniak API to sync subscription status. Something similar could be built for any third-party newsletter service that offers an API.