If you're having trouble verifying your Namecheap domain on Mailgun, here's the working version:

It took minutes for these DNS changes to take effect instead of 24-48 hours suggested by Mailgun once I got the settings right on the "Advanced DNS" page.

If you're getting "Domain not found: ..." and your domain has a little EU flag next to it in the Mailgun Dashboard, try changing the API host to EU:

const auth = {
  auth: {
    api_key: ...,
    domain: 'mg.yourdomein.com'
  },
  host: 'api.eu.mailgun.net',
}