FEATURE: erode bounce score every time an email is sent

Introduces a hidden setting (default is 0.1) that erodes bounce score
every time we send an email. This means that erratic failures are less
painful cause system auto corrects

app/jobs/regular/user_email.rb

@@ -45,6 +45,12 @@ module Jobs
 
       if message
         Email::Sender.new(message, type, user).send
+        if (b = user.user_stat.bounce_score) > SiteSetting.bounce_score_erode_on_send
+          # erode bounce score each time we send an email
+          # this means that we are punished a lot less for bounces
+          # and we can recover more quickly
+          user.user_stat.update(bounce_score: b - SiteSetting.bounce_score_erode_on_send)
+        end
       else
         skip_reason_type
       end

config/site_settings.yml

@@ -841,6 +841,9 @@ email:
     default: 4
     min: 1
   bounce_score_threshold_deactivate: 30
+  bounce_score_erode_on_send:
+    default: 0.1
+    hidden: true
   soft_bounce_score:
     default: 1
     min: 1

db/migrate/20180828065005_change_bounce_score_to_float.rb

@@ -0,0 +1,5 @@
+class ChangeBounceScoreToFloat < ActiveRecord::Migration[5.2]
+  def change
+    change_column :user_stats, :bounce_score, :float
+  end
+end

spec/jobs/user_email_spec.rb

@@ -291,6 +291,34 @@ describe Jobs::UserEmail do
         end
       end
 
+      it "erodes bounce score each time an email is sent" do
+        SiteSetting.bounce_score_erode_on_send = 0.2
+
+        user.user_stat.update(bounce_score: 2.7)
+
+        Jobs::UserEmail.new.execute(
+          type: :user_mentioned,
+          user_id: user.id,
+          notification_id: notification.id,
+          post_id: post.id
+        )
+
+        user.user_stat.reload
+        expect(user.user_stat.bounce_score).to eq(2.5)
+
+        user.user_stat.update(bounce_score: 0)
+
+        Jobs::UserEmail.new.execute(
+          type: :user_mentioned,
+          user_id: user.id,
+          notification_id: notification.id,
+          post_id: post.id
+        )
+
+        user.user_stat.reload
+        expect(user.user_stat.bounce_score).to eq(0)
+      end
+
       it "does not send notification if bounce threshold is reached" do
         user.user_stat.update(bounce_score: SiteSetting.bounce_score_threshold)