I had similiar issues setting up my production instance to work with mailgun. I think nodemailer might have an issue with the single quotes in the from field. I did some research back then and if I remember correctly, the single quotes aren’t explicitly necessary. After I got rid of the single quotes it started working. Here’s the
mail portion of the config used by the instance (the sender works properly):
"from": "My Ghost Robot <firstname.lastname@example.org>",
With regards to SPF / DKIM, Dmarcian is a well known service which provides inspection of DMARC (it’s a txt record that you add to your domain which configures reporting for DKIM + SPF), and it looks like mailgun isn’t permitted to send (it only allows zoho) via your spf records. You can run your check here. I don’t know what selector you chose for dkim, so I can’t verify it’s setup properly.
You’re sending the test email using the labs page in the admin interface, correct?