diff --git a/app/models/dmail.rb b/app/models/dmail.rb
index 4e861d5d1..0bfcb7eb0 100644
--- a/app/models/dmail.rb
+++ b/app/models/dmail.rb
@@ -1,6 +1,11 @@
 require 'digest/sha1'
 
 class Dmail < ApplicationRecord
+  # if a person sends spam to more than 10 users within a 24 hour window, automatically ban them for 3 days.
+  AUTOBAN_THRESHOLD = 10
+  AUTOBAN_WINDOW = 24.hours
+  AUTOBAN_DURATION = 3
+
   include Rakismet::Model
 
   with_options on: :create do
@@ -23,6 +28,23 @@ class Dmail < ApplicationRecord
   rakismet_attrs author: :from_name, author_email: :from_email, content: :title_and_body, user_ip: :creator_ip_addr_str
 
   concerning :SpamMethods do
+    class_methods do
+      def is_spammer?(user)
+        return false if user.is_gold?
+
+        spammed_users = sent_by(user).where(is_spam: true).where("created_at > ?", AUTOBAN_WINDOW.ago).distinct.count(:to_id)
+        spammed_users >= AUTOBAN_THRESHOLD
+      end
+
+      def ban_spammer(spammer)
+        spammer.bans.create! do |ban|
+          ban.banner = User.system
+          ban.reason = "Spambot."
+          ban.duration = AUTOBAN_DURATION
+        end
+      end
+    end
+
     def title_and_body
       "#{title}\n\n#{body}"
     end
@@ -31,9 +53,9 @@ class Dmail < ApplicationRecord
       creator_ip_addr.to_s
     end
 
-    def spam?(sender = CurrentUser.user)
+    def spam?
       return false if Danbooru.config.rakismet_key.blank?
-      return false if sender.is_gold?
+      return false if from.is_gold?
       super()
     end
   end
@@ -58,7 +80,6 @@ class Dmail < ApplicationRecord
     def initialize_attributes
       self.from_id ||= CurrentUser.id
       self.creator_ip_addr ||= CurrentUser.ip_addr
-      self.is_spam = spam?(CurrentUser.user)
     end
   end
 
@@ -73,6 +94,7 @@ class Dmail < ApplicationRecord
           # recipient's copy
           copy = Dmail.new(params)
           copy.owner_id = copy.to_id
+          copy.is_spam = copy.spam?
           copy.save unless copy.to_id == copy.from_id
 
           # sender's copy
@@ -80,13 +102,15 @@ class Dmail < ApplicationRecord
           copy.owner_id = copy.from_id
           copy.is_read = true
           copy.save
+
+          Dmail.ban_spammer(copy.from) if Dmail.is_spammer?(copy.from)
         end
 
         copy
       end
 
       def create_automated(params)
-        dmail = Dmail.new(from: Danbooru.config.system_user, **params)
+        dmail = Dmail.new(from: User.system, **params)
         dmail.owner = dmail.to
         dmail.save
         dmail
@@ -119,6 +143,10 @@ class Dmail < ApplicationRecord
   end
   
   module SearchMethods
+    def sent_by(user)
+      where("dmails.from_id = ? AND dmails.owner_id != ?", user.id, user.id)
+    end
+
     def active
       where("is_deleted = ?", false)
     end
@@ -234,7 +262,7 @@ class Dmail < ApplicationRecord
   end
 
   def is_automated?
-    from == Danbooru.config.system_user
+    from == User.system
   end
 
   def filtered?
diff --git a/app/models/user.rb b/app/models/user.rb
index ca582c23f..be950b7f8 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -316,7 +316,7 @@ class User < ApplicationRecord
 
     module ClassMethods
       def system
-        Danbooru.config.system_user
+        User.find_by!(name: Danbooru.config.system_user)
       end
 
       def level_hash
@@ -366,7 +366,7 @@ class User < ApplicationRecord
     def promote_to_admin_if_first_user
       return if Rails.env.test?
 
-      if User.count == 0
+      if User.admins.count == 0
         self.level = Levels::ADMIN
         self.can_approve_posts = true
         self.can_upload_free = true
diff --git a/config/danbooru_default_config.rb b/config/danbooru_default_config.rb
index ec254f56a..874eef4a6 100644
--- a/config/danbooru_default_config.rb
+++ b/config/danbooru_default_config.rb
@@ -37,8 +37,10 @@ module Danbooru
     end
 
     # System actions, such as sending automated dmails, will be performed with this account.
+    # This account will be created automatically if it doesn't exist. It will
+    # be promoted to Moderator if it isn't already a Moderator.
     def system_user
-      User.find_by_name("DanbooruBot") || User.admins.first
+      "DanbooruBot"
     end
 
     def upload_feedback_topic
diff --git a/config/initializers/system_user.rb b/config/initializers/system_user.rb
new file mode 100644
index 000000000..2c210facd
--- /dev/null
+++ b/config/initializers/system_user.rb
@@ -0,0 +1,10 @@
+require "securerandom"
+
+system = User.find_or_create_by!(name: Danbooru.config.system_user) do |user|
+  user.password = SecureRandom.base64(32)
+end
+
+unless system.is_moderator?
+  system.level = User::Levels::MODERATOR
+  system.save
+end
diff --git a/test/unit/dmail_test.rb b/test/unit/dmail_test.rb
index 0d4427cce..7e40ffcd9 100644
--- a/test/unit/dmail_test.rb
+++ b/test/unit/dmail_test.rb
@@ -3,6 +3,7 @@ require 'test_helper'
 class DmailTest < ActiveSupport::TestCase
   context "A dmail" do
     setup do
+      User.any_instance.stubs(:validate_sock_puppets).returns(true)
       @user = FactoryGirl.create(:user)
       CurrentUser.user = @user
       CurrentUser.ip_addr = "1.2.3.4"
@@ -23,10 +24,24 @@ class DmailTest < ActiveSupport::TestCase
 
       should "not validate" do
         assert_difference("Dmail.count", 2)do
-          dmail = Dmail.create_split(:to_id => @recipient.id, :title => "My video", :body => "hey Noneeditsonlyme.  My webcam see here http://bit.ly/2vTv9Ki")
-          assert(dmail.is_spam?)
+          Dmail.create_split(:to_id => @recipient.id, :title => "My video", :body => "hey Noneeditsonlyme.  My webcam see here http://bit.ly/2vTv9Ki")
+          assert(@recipient.dmails.last.is_spam?)
         end
       end
+
+      should "autoban spammers after sending spam to N distinct users" do
+        Dmail.any_instance.expects(:spam?).returns(true)
+
+        users = FactoryGirl.create_list(:user, Dmail::AUTOBAN_THRESHOLD)
+        users.each do |user|
+          Dmail.create_split(from: @user, to: user, title: "spam", body: "wonderful spam")
+        end
+
+        assert_equal(true, Dmail.is_spammer?(@user))
+        assert_equal(true, @user.reload.is_banned)
+        assert_equal(1, @user.bans.count)
+        assert_match(/Spambot./, @user.bans.last.reason)
+      end
     end
 
     context "filter" do
@@ -177,7 +192,7 @@ class DmailTest < ActiveSupport::TestCase
     context "that is automated" do
       setup do
         @bot = FactoryGirl.create(:user)
-        Danbooru.config.stubs(:system_user).returns(@bot)
+        User.stubs(:system).returns(@bot)
       end
 
       should "only create a copy for the recipient" do
diff --git a/test/unit/post_disapproval_test.rb b/test/unit/post_disapproval_test.rb
index 8b010d8f8..c4ef31399 100644
--- a/test/unit/post_disapproval_test.rb
+++ b/test/unit/post_disapproval_test.rb
@@ -85,7 +85,7 @@ class PostDisapprovalTest < ActiveSupport::TestCase
 
         should "dmail the uploaders" do
           bot = FactoryGirl.create(:user)
-          Danbooru.config.stubs(:system_user).returns(bot)
+          User.stubs(:system).returns(bot)
 
           assert_difference(["@uploaders[0].dmails.count", "@uploaders[1].dmails.count"], 1) do
             PostDisapproval.dmail_messages!
diff --git a/test/unit/post_replacement_test.rb b/test/unit/post_replacement_test.rb
index e33fdbf4a..3785bafea 100644
--- a/test/unit/post_replacement_test.rb
+++ b/test/unit/post_replacement_test.rb
@@ -20,7 +20,7 @@ class PostReplacementTest < ActiveSupport::TestCase
     Delayed::Worker.delay_jobs = true # don't delete the old images right away
 
     @system = FactoryGirl.create(:user, created_at: 2.weeks.ago)
-    Danbooru.config.stubs(:system_user).returns(@system)
+    User.stubs(:system).returns(@system)
 
     @uploader = FactoryGirl.create(:user, created_at: 2.weeks.ago, can_upload_free: true)
     @replacer = FactoryGirl.create(:user, created_at: 2.weeks.ago, can_approve_posts: true)
diff --git a/test/unit/user_test.rb b/test/unit/user_test.rb
index 8cd53246e..21efdb64c 100644
--- a/test/unit/user_test.rb
+++ b/test/unit/user_test.rb
@@ -28,7 +28,7 @@ class UserTest < ActiveSupport::TestCase
 
       should "send an automated dmail to the user" do
         bot = FactoryGirl.create(:user)
-        Danbooru.config.stubs(:system_user).returns(bot)
+        User.stubs(:system).returns(bot)
 
         assert_difference("Dmail.count", 1) do
           @user.promote_to!(User::Levels::GOLD)