TMS Email API SG is a FileMaker app to connect with the SendGrid API, which is a commercial and paid email service. SendGrid is a communication platform for transactional and marketing email, and is owned by Twilio.
SendGrid allows you to send group emails and track statistics about them such as how many were sent, how many were received, how many were opened and how many were clicked through, thereby letting you evaluate the effectiveness of an email campaign. This solution works for for everything from very large marketing emails to small non-profit newsletters. It even works fine for individual emails.
You can either populate this template with your emails or you can copy and integrate the script into your own custom app. This solution is completely open for you to customize as necessary for your FileMaker solution.