Everyone.
Just kidding. Maybe 
The short answer is that you should be posting it to everyone that’s on your current Prospect List and ideally, also your clients.
In the early stages of marketing, your Prospect List might only include people you’ve met at events or people who have asked you for quotes in the past but haven’t become clients yet.
However over time, you should end up buying / building & managing a much larger prospect list of all your ideal clients who you will send Printed Newsletters in conjunction with many other things such as regular Direct Mail campaigns and Postcards.
Whilst buying / building your Prospect List is a topic for another day, know that there are lots of MSPs out there successfully buying lists and sending Printed Newsletters & Direct Mail pieces to them and getting lots of leads & clients from doing it consistently.