Mailgun Sandbox domain not working

Question

Net::SMTPFatalError (554 Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authorized recipients in domain settings.

I am using the Mailgun addon with Heroku in a RoR application. I am trying to use the sandbox domain with my personal gmail account added as an authorized user. I am using devise confirmable, so when a new user signs up, a default email should be sent to the email they provided. As seen below, the user is successfully being created and the email to be sent is found. Mailgun seems to recieve the necessary information, but does not believe I am authorized?

    SQL (1.1ms)  INSERT INTO "users" ("first_name", "last_name", "email", "encrypted_password", "created_at", "updated_at", "confirmation_token", "confirmation_sent_at") VALUES ($1, $2, $3, $4, $5, $6, $7, $8) RETURNING "id"  [["first_name", "C"], ["last_name", "C"], ["email", "[email protected]"], ["encrypted_password", "$2a$10$k9CdjjFd7GzLcKJ.E4VNie27aJrQm3LnigLHlFSFKcd2qR2x11cQW"], ["created_at", "2016-08-01 22:05:30.331566"], ["updated_at", "2016-08-01 22:05:30.331566"], ["confirmation_token", "38f6d620e0e8277957ee85bbede77610cdbea448a79b9d07fc5998fdc3c780d3"], ["confirmation_sent_at", "2016-08-01 22:05:30.565564"]]
2016-08-01T22:05:30.601828+00:00 app[web.1]:   Rendered vendor/bundle/ruby/2.2.0/gems/devise-3.4.1/app/views/devise/mailer/confirmation_instructions.html.erb (9.0ms)
2016-08-01T22:05:30.856455+00:00 app[web.1]: 2016-08-01T22:05:30.856470+00:00 app[web.1]: MyMailer#confirmation_instructions: processed outbound mail in 284.9ms
2016-08-01T22:05:30.978021+00:00 app[web.1]: 
2016-08-01T22:05:30.978023+00:00 app[web.1]: Sent mail to [email protected] (121.4ms)
2016-08-01T22:05:30.978026+00:00 app[web.1]: Date: Mon, 01 Aug 2016 22:05:30 +0000
2016-08-01T22:05:30.978027+00:00 app[web.1]: From: [email protected]
2016-08-01T22:05:30.978028+00:00 app[web.1]: To: [email protected]
2016-08-01T22:05:30.978028+00:00 app[web.1]: Message-ID: <579fc7aad20f8_33f9a82512758405f4@e15a6ab3-869e-4811-9cde-20f13e3e0db4.mail>
2016-08-01T22:05:30.978029+00:00 app[web.1]: Subject: Confirmation instructions
2016-08-01T22:05:30.978029+00:00 app[web.1]: Mime-Version: 1.0
2016-08-01T22:05:30.978030+00:00 app[web.1]: Content-Type: text/html;
2016-08-01T22:05:30.978030+00:00 app[web.1]:  charset=UTF-8
2016-08-01T22:05:30.978030+00:00 app[web.1]: Content-Transfer-Encoding: 7bit
2016-08-01T22:05:30.978031+00:00 app[web.1]: 
2016-08-01T22:05:30.978031+00:00 app[web.1]: <p>Welcome [email protected]!</p>
2016-08-01T22:05:30.978032+00:00 app[web.1]: 
2016-08-01T22:05:30.978034+00:00 app[web.1]: <p>You can confirm your account email through the link below:</p>
2016-08-01T22:05:30.978034+00:00 app[web.1]: 
2016-08-01T22:05:30.978035+00:00 app[web.1]: <p><a href="http://peeraccomplish.heroku.com/users/confirmation?confirmation_token=zry54mp6sBgdFdfcjPYP">Confirm my account</a></p>
2016-08-01T22:05:30.978036+00:00 app[web.1]: 
2016-08-01T22:05:30.979055+00:00 app[web.1]:    (0.9ms)  ROLLBACK
2016-08-01T22:05:30.981993+00:00 app[web.1]: Completed 500 Internal Server Error in 753ms
2016-08-01T22:05:30.983041+00:00 app[web.1]: 
2016-08-01T22:05:30.983042+00:00 app[web.1]: ):
Net::SMTPFatalError (554 Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authorized recipients in domain settings.

I used the directions on using Mailgun for Heroku - (copied and pasted into my config/production.rb):

ActionMailer::Base.smtp_settings = {
    :port           => ENV['MAILGUN_SMTP_PORT'],
    :address        => ENV['MAILGUN_SMTP_SERVER'],
    :user_name      => ENV['MAILGUN_SMTP_LOGIN'],
    :password       => ENV['MAILGUN_SMTP_PASSWORD'],
    :domain         => ENV['MAILGUN_DOMAIN'],
    :authentication => :plain,
  }
  ActionMailer::Base.delivery_method = :smtp

I have checked the environment variables and they all appear accurate. I also checked port 587 is working using telnet on heroku.

Any ideas where to go from here?

mailers/my_mailer.rb for reference:

class MyMailer < Devise::Mailer   
  helper :application # gives access to all helpers defined within `application_helper`.
  include Devise::Controllers::UrlHelpers # Optional. eg. `confirmation_url`
  default template_path: 'devise/mailer' # to make sure that your mailer uses the devise views

  default from: '[email protected]'
end

Answer 1

I had this issue too. The problem is, when using the mailgun sandbox domain, you have to pre-register your recipient e-mail in order to send emails to it. In order to do this, go to your app mailgun dashboard, click in "authorized recipients", like the image below.

Where to go to add an authorized recipient

Answer 2

I believe this was an issue on Mailgun's end. I confirmed that I had an authorized user associated with the Sandbox domain. Having no luck with the Sandbox domain, I registered a domain, and updated the config vars in the Heroku app settings, and with no changes to the code above, got emails sending.