Use your forms action attribute to submit to your custom endpoint. That's all.
Javascript forms
Via command line
curl is your friend? Just call the custom endpoint. It's a cinch.
Decide on your backend
Use the mailing service you like. May it be for privacy or cost reaons. We support multiple different providers. Want to use your own SMTP server? Just do it!
Store or proxy
Decide on whether you want the messages to be saved or just forwarded. Configure it for each form individually.