Allow to set SMTP settings via environment variable.

config/environments/production.rb.sample

@@ -40,8 +40,27 @@ Discourse::Application.configure do
   #   :authentication       => 'plain',
   #   :enable_starttls_auto => true  }
 
-  config.action_mailer.delivery_method = :sendmail
-  config.action_mailer.sendmail_settings = {arguments: '-i'}
+  if ENV.key?('SMTP_URL')
+    config.action_mailer.smtp_settings = begin
+      uri = URI.parse(ENV['SMTP_URL'])
+      params = {
+        :address              => uri.host,
+        :port                 => uri.port,
+        :domain               => (uri.path || "").split("/")[1],
+        :user_name            => uri.user,
+        :password             => uri.password,
+        :authentication       => 'plain',
+        :enable_starttls_auto => true
+      }
+      CGI.parse(uri.query || "").each {|k,v| params[k.to_sym] = v.first}
+      params
+    rescue
+      raise "Invalid SMTP_URL"
+    end
+  else
+    config.action_mailer.delivery_method = :sendmail
+    config.action_mailer.sendmail_settings = {arguments: '-i'}
+  end
 
   # Send deprecation notices to registered listeners
   config.active_support.deprecation = :notify