This is a step by step blog to guide you how to set up Mailgun sending API keys for your domain and set it up in your self hosted Ghost blog.
Setup Sending API key in Mailgun
login into your Mailgun account, go through the menu steps:
Dashboard -> Sending -> Domain settings -> Sending API keys
Click 'Add sending key'
Input a simple description and click 'Create sending key'
Copy the Sending API key into clipboard
Ghost Email Newsletter Setting
Copy the API key you just created and pasted as Mailgun Private API key.
Select correct Mailgun region and Input Mailgun domain.
PS: please don't include https:// for your domain. Just put the value you have on your Mailgun dashboard. I smartass added some prefixes that caused this setup to be stuck there for days.:(