title: Postmark mailing working only on production env
body: <p>I have three environments and postmark mailing is just working on <code>production</code>. I checked the api keys that I created and nothing. I even tried to use the same api key from production and it doesn't work on staging. </p>
  
  <pre><code>Rails v. 5.0.1
  </code></pre>
  
  <p>gemfile:</p>
  
  <pre><code>gem 'postmark' (v.1.14.0)
  
  gem 'postmark-rails' (v. 0.18.0)
  </code></pre>
  
  <p><strong>production.rb</strong></p>
  
  <pre><code>  config.action_mailer.delivery_method = :postmark
    config.action_mailer.postmark_settings = { :api_token =&gt; ENV["POSTMARK_API_KEY"] }
    config.action_mailer.default_url_options = { host: "www.mywebsite.com" }
  </code></pre>
  
  <p><strong>development.rb</strong></p>
  
  <pre><code>  config.action_mailer.delivery_method = :postmark
    config.action_mailer.postmark_settings = { :api_token =&gt; ENV["POSTMARK_API_KEY"] }
    config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }
  </code></pre>
  
  <p><strong>staging.rb</strong></p>
  
  <pre><code>  config.action_mailer.delivery_method = :postmark
    config.action_mailer.postmark_settings = { :api_token =&gt; ENV["POSTMARK_API_KEY"] }
    config.action_mailer.default_url_options = { host: "myapp.herokuapp.com" }
  </code></pre>