prevent the application from starting in development mode when SMTP settings are changed

config/environment.rb

@@ -3,3 +3,8 @@ require File.expand_path('../application', __FILE__)
 
 # Initialize the rails application
 Discourse::Application.initialize!
+
+# When in "dev" mode, ensure we won't be sending any emails
+if Rails.env.development? && ActionMailer::Base.smtp_settings != { address: "localhost", port: 1025 }
+  fail "In development mode, you should be using mailcatcher otherwise you might end up sending thousands of digest emails"
+end