Formspree.io is an online service to provide form processing facilities to static front-end only websites. It's an excellent tool for adding simple contact forms. Setup is quick and I've found the delivery mechanism to be reliable.
The service provides a back-end email delivery system for form submissions. An email message is built from the form data and sent to a specificed email address. The form structure is simple and to use it you simply include the provided form code in the website's HTML.
Here's an example taken from the Formspree website of what the HTML form code looks like:
<form action="https://firstname.lastname@example.org" method="POST"> <input type="text" name="name"> <input type="email" name="_replyto"> <input type="submit" value="Send"> </form>
When the code is added with the relevant email address, on first submission of the form, Formspree will send a confirmation email to the destination email address. Once confirmation has been received, all form submissions will send the form data to the destination address as an email.
It is free to use for up to one-thousand email submissions per month. For usage levels above this, there is the unlimited Gold Plan option which costs $9.99 per month (at time of writing).
However, one note of caution should be noted. This approach is not suitable for use in situations where the data being transmitted should be confidential and secure. Formspree say that their policy is to "store the last one-hundred form submissions and delete the rest. Emails are sent using SendGrid's API, so on that end their privacy policies apply." Obviously, the fact that the confidentially of the information sent is outside of your control means that you should carefully consider whether this approach is appropriate for your customer's needs.
To find out more, visit the Formspree.io website.