Mystery Solved: Open email tracking works!
A big difference between Ghost and Wordpress is the bundled functionality. Wordpress’ position is to just toss the capability in there, or to have plugins that do the work, and assume that works for everybody. And it usually does. Ghost on the other hand is leaner, and uses outside integrations. Want your posts to automatically get posted to Twitter? Use Zapier. Sending emails? Integrate with Mailgun.
I have three Ghost blogs, and one Mailgun account (my reach is so small, I fit comfortably within the “Free” tier of service). The first one I set up, to email people directly on posts tracked opens of emails. The other two? Not so successful.
Turns out that I created API credentials for the two other domains, and used them for setting up the newsletter functionality. And that was the reason for it not working.
All I had to do was to use the master account private API key, and voilá, open tracking works.
I swear I googled the fuck out of this issue with no luck, until I found an obscure post to a forum.
Operator error, as usual