Add email interceptor to prevent mail sending if email sending is disabled.

config/initializers/disable_email_interceptor.rb

+# Interceptor in lib/disable_email_interceptor.rb
+ActionMailer::Base.register_interceptor(DisableEmailInterceptor) unless Gitlab.config.gitlab.email_enabled

lib/disable_email_interceptor.rb

+# Read about interceptors in http://guides.rubyonrails.org/action_mailer_basics.html#intercepting-emails
+class DisableEmailInterceptor
+
+  def self.delivering_email(message)
+    message.perform_deliveries = false
+    Rails.logger.info "Emails disabled! Interceptor prevented sending mail #{message.subject}"
+  end
+end