Email marketing/updates questions

Hi,

I want to reach out to people who've signed up for my products with updates, service announcements, usage tips and tricks, etc.

I didn't think to put a "checkbox" for getting their permission for email updates, so what do I need to do here? People that have made purchases I can implicitly consider opted in, but what about free users?

What's the norms here? What do I need to know? What services can I use? Email Octopus, ConvertKit, something else?

Would really be super grateful for some pointers here.