Zapier itself is not an email service like SendGrid. Zapier is more of a service that is used to add additional functionality to your community when an action on your community or another application integrated with Zapier is performed. It, by itself, is not an email sending service.
There are hundreds of email sending services out there which work over SMTP or have APIs. If you are looking specifically for an API for a reason, you may need to hire a developer as currently the only integration in the core software is SendGrid.
Some other email services are like AWS' SES, Sparkpost, SocketLabs, etc...