Visit Mailgun.com and click Sign Up. Fill in the form. It is important that you leave 'Add payment info now' checked: to be able to send emails, you need to attach your credit card to your Mailgun's account. In the dropdown menu, choose 'Pay as you go' as your plan. You will not be billed unless you send more than 10,000 emails per month. Click Create Account.
2. Then check your inbox to verify your Mailgun account by clicking the link.
3. Click Sending → Add Your Domain and enter your domain name.
Then click Overview and follow the instructions on how to verify your domain name.
Click Overview again → API and find your API Key.
Copy your API Key and paste into the plugin settings with your domain name.
Finish by clicking Save Changes.