Show a warning if there are email jobs in the retry queue

app/models/admin_dashboard_data.rb

@@ -40,7 +40,15 @@ class AdminDashboardData
   end
 
   def problems
-    [rails_env_check, host_names_check, gc_checks, sidekiq_check || queue_size_check || clockwork_check, ram_check, facebook_config_check, twitter_config_check, github_config_check].compact
+    [ rails_env_check,
+      host_names_check,
+      gc_checks,
+      sidekiq_check || queue_size_check || clockwork_check,
+      ram_check,
+      facebook_config_check,
+      twitter_config_check,
+      github_config_check,
+      failing_emails_check ].compact
   end
 
   def rails_env_check
@@ -84,4 +92,9 @@ class AdminDashboardData
   def github_config_check
     I18n.t('dashboard.github_config_warning') if SiteSetting.enable_github_logins and (!SiteSetting.github_client_id.present? or !SiteSetting.github_client_secret.present?)
   end
+
+  def failing_emails_check
+    num_failed_jobs = Jobs.num_email_retry_jobs
+    I18n.t('dashboard.failing_emails_warning', num_failed_jobs: num_failed_jobs) if num_failed_jobs > 0
+  end
 end

config/locales/server.en.yml

@@ -346,6 +346,7 @@ en:
     facebook_config_warning: 'The server is configured to allow signup and log in with Facebook (enable_facebook_logins), but the app id and app secret values are not set. Go to <a href="/admin/site_settings">the Site Settings</a> and update the settings. <a href="https://github.com/discourse/discourse/wiki/The-Discourse-Admin-Quick-Start-Guide#enable-facebook-logins" target="_blank">See this guide to learn more</a>.'
     twitter_config_warning: 'The server is configured to allow signup and log in with Twitter (enable_twitter_logins), but the key and secret values are not set. Go to <a href="/admin/site_settings">the Site Settings</a> and update the settings. <a href="https://github.com/discourse/discourse/wiki/The-Discourse-Admin-Quick-Start-Guide#enable-twitter-logins" target="_blank">See this guide to learn more</a>.'
     github_config_warning: 'The server is configured to allow signup and log in with GitHub (enable_github_logins), but the client id and secret values are not set. Go to <a href="/admin/site_settings">the Site Settings</a> and update the settings. <a href="https://github.com/discourse/discourse/wiki/The-Discourse-Admin-Quick-Start-Guide" target="_blank">See this guide to learn more</a>.'
+    failing_emails_warning: "There are %{num_failed_jobs} email jobs that failed. Check your config/production.rb file and ensure that the config.action_mailer settings are correct."
 
   content_types:
     education_new_reply:

lib/jobs.rb

@@ -11,6 +11,10 @@ module Jobs
     end
   end
 
+  def self.num_email_retry_jobs
+    Sidekiq::RetrySet.new.select { |job| job.klass =~ /Email$/ }.size
+  end
+
   class Base
     include Sidekiq::Worker