FlyingForms is providing a random sender adress for an easy start to use the mail submit actions. To define your own sender adress, a Mail Setting is needed.
Go to https://flyingforms.io/projects/[YOUR_PROJECT]/Forms/MailSettings/Index and create a new mail setting. Next to the basic SMTP setting, further external providers are available.
After filling the fields, a test mail can be send. It's a generic test mail to verify, that the settings are correct.
The mail submit actions in the form designer can select Mail Settings.