Every message being throttled

Hi, I'm new at using logstash. I'm trying to use throttling in my filter and every message is getting throttled for some reason. Do I have an error in my config?

I was expecting every 2nd, 3rd, and 4th message containing "STUFF" to have the tag "SEND_EMAIL" added to it. But instead every message has the tag "throttled".

if "STUFF" in [message] {
    throttle {
      before_count => 2
      after_count => 4
      period => 600
      key => "%{message}"
      add_tag => "throttled"

if "throttled" not in [tags] {
   mutate {
      add_tag => "SEND_EMAIL"