Mailgun Error

Mailgun Code

irb(main):012:0> 
irb(main):013:0* RestClient.post "https://api:#{API_KEY}@api.mailgun.net/v2/mailgun.net/messages", 
irb(main):014:0*   :from => "[email protected]",
irb(main):015:0*   :to => '[email protected]',
irb(main):016:0*   :subject => "This is subject",
irb(main):017:0*   :text => "Text body",
irb(main):018:0*   :html => "<b>HTML</b> version of the body!"
RestClient::ResourceNotFound: 404 Resource Not Found: {
  "message": "Domain not found: mailgun.net"
}