Failed to send newsletter

I have added my mailgun information in the settings > Email newsletter in Ghost.
I have a free plan on mailgun. I have 5 subscribers.
Sending a test email doesn’t seem to get delivered, but it says “sent”.
Edit: The test email gets delivered, but is delivered in the spam folder (any fix for that?)
Whenever a new member has subscribed or sending emails like a subscription confirmation do work and get delivered.

Is there a way to fix this? Am I doing something wrong?