Dumb question for a SAAS MVP as a non programmer..

Sounds like you already got the authentication piece sorted out. If that's the case I'd agree with @Johnkueh and use Sendgrid.

But I'd highly recommend looking at Auth0 to handle all of your authentication. They have a really solid product, and you get a lot on their free plan.

Firebase

Hey there. At https://www.bankpayments.net we use auth0.com for our signup flow and authentication. Its really good for the use cases you described.

Disclaimer: I have used neither of these services but if I were to I would look at using Firebase Authentication or Netlify Identity. Live demo of Netlify's offering here: https://github.com/netlify/netlify-identity-widget#netlify-identity-widget

Sendgrid/SES + Nodemailer + email-templates

Funnily enough I wrote a post about my experiences with this yesterday.

It depends what you want your app to do, but the best platform I've tried is Bubble. Super quick to get up and running and good value.

Little post here about how I did my first one and the basics: https://prodcel.com/blog/building-an-advanced-web-app-with-no-code/

It all depends on what you actually use for your SaaS MVP.