RESOLVED:
Apparently, there are two separate ways to create API keys in the Mailgun service as follows:
- Through Sending > Domain Settings > Sending API keys
- Through Account > API Keys
This isn’t clear in the Ghost documentation which one is required, although the link in the Ghost UI does take you to the correct screen in Mailgun but I didn’t use that link, I navigated in the Mailgun UI to the newer version of API keys. The one Ghost is using is the v3.
#1 above allows you to create individual keys for the v4 service for each of your domains.
#2 is an overall private API key that is used for the entire Mailgun account. Their v3 service.
Ghost wants #2. I was using #1.
All set now. It works.