Transactional emails on a static site without a backend?

Question

API

What services are suitable for me to create a simple form on a static site (filling in a couple of fields + sending an email confirming the submission of the form + marketing mailing)?
Mailchimp, Sendgrid, Mailgun etc? It's hard to get them all. Who used, which of them have a solution that suits me?
Maybe there are JS libraries to do everything on the client side?
And how, in a nutshell, does the implementation of this task look like?

Answer 1

Is the API key also on the client side?
GoogleForms look

Answer 2

There are examples of ajax sending here:
stackoverflow.com/questions/7381150/how-to-send-an...