You can do your own google searching, but in my opinion Substack has the worst smell of email newsletter providers.
An app's smell is the vibe of the thing. Pretty self explanatory.
I don't have time to explain why these apps below are better than Substack because I'm such a prolific writer, but please use any of these apps before you consider sending out your email newsletter before you end up on Substack:
The only one of the above I have some sort of referral link built in is Buttondown because it's the one I currently use to rarely send out emails to 30 some subscribers. I have tried all of the above apps for sending newsletters, including Substack, and any of the 4 apps I listed care more about your writing than Substack actually does - and they don't smell bad.
That's one of the best tips I have actually: you don't have to sign up for Substack just to figure out you're not able to reliably send out an email newsletter. You can do that with any service, so why not use one that doesn't have an annual controversy or sleazy ownership.