diff --git a/app/controllers/admin/email_templates_controller.rb b/app/controllers/admin/email_templates_controller.rb
index b32f0b919..e7d632580 100644
--- a/app/controllers/admin/email_templates_controller.rb
+++ b/app/controllers/admin/email_templates_controller.rb
@@ -11,7 +11,7 @@ class Admin::EmailTemplatesController < Admin::AdminController
                      "system_messages.email_reject_destination", "system_messages.email_reject_empty",
                      "system_messages.email_reject_invalid_access", "system_messages.email_reject_no_account",
                      "system_messages.email_reject_parsing", "system_messages.email_reject_post_error",
-                     "system_messages.email_reject_post_error_specified",
+                     "system_messages.email_reject_post_error_specified", "system_messages.email_reject_user_not_found",
                      "system_messages.email_reject_reply_key", "system_messages.email_reject_topic_closed",
                      "system_messages.email_reject_topic_not_found", "system_messages.email_reject_trust_level",
                      "system_messages.pending_users_reminder", "system_messages.post_hidden",
diff --git a/app/jobs/scheduled/poll_mailbox.rb b/app/jobs/scheduled/poll_mailbox.rb
index c01da2d2d..03193837f 100644
--- a/app/jobs/scheduled/poll_mailbox.rb
+++ b/app/jobs/scheduled/poll_mailbox.rb
@@ -40,6 +40,7 @@ module Jobs
       message_template = case e
         when Email::Receiver::EmptyEmailError             then :email_reject_empty
         when Email::Receiver::NoBodyDetectedError         then :email_reject_empty
+        when Email::Receiver::UserNotFoundError           then :email_reject_user_not_found
         when Email::Receiver::AutoGeneratedEmailError     then :email_reject_auto_generated
         when Email::Receiver::InactiveUserError           then :email_reject_inactive_user
         when Email::Receiver::BlockedUserError            then :email_reject_blocked_user
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index c293a5b58..04bd73c1c 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -1162,6 +1162,7 @@ en:
 
     delete_email_logs_after_days: "Delete email logs after (N) days. 0 to keep indefinitely"
     max_emails_per_day_per_user: "Maximum number of emails to send users per day. 0 to disable the limit"
+    enable_staged_users: "Automatically create staged users when processing incoming emails."
 
     manual_polling_enabled: "Push emails using the API for email replies."
     pop3_polling_enabled: "Poll via POP3 for email replies."
@@ -1828,6 +1829,13 @@ en:
 
         Your account does not have the required trust level to post new topics to this email address. If you believe this is in error, contact a staff member.
 
+    email_reject_user_not_found:
+      subject_template: "[%{site_name}] Email issue -- User Not Found"
+      text_body_template: |
+        We're sorry, but your email message to %{destination} (titled %{former_title}) didn't work.
+
+        Your reply was sent from an unknown email address. Try sending from another email address, or contact a staff member.
+
     email_reject_inactive_user:
       subject_template: "[%{site_name}] Email issue -- Inactive User"
       text_body_template: |
diff --git a/config/site_settings.yml b/config/site_settings.yml
index 096b4eab5..4d516bc1e 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -560,6 +560,7 @@ email:
     default: 365
     min: 0
   max_emails_per_day_per_user: 100
+  enable_staged_users: true
 
 files:
   max_image_size_kb: 3072
diff --git a/lib/email/receiver.rb b/lib/email/receiver.rb
index 5c16c7e09..2211541e1 100644
--- a/lib/email/receiver.rb
+++ b/lib/email/receiver.rb
@@ -10,6 +10,7 @@ module Email
 
     class ProcessingError             < StandardError; end
     class EmptyEmailError             < ProcessingError; end
+    class UserNotFoundError           < ProcessingError; end
     class AutoGeneratedEmailError     < ProcessingError; end
     class NoBodyDetectedError         < ProcessingError; end
     class InactiveUserError           < ProcessingError; end
@@ -53,6 +54,9 @@ module Email
 
     def process_internal
       user = find_or_create_user(@from_email, @from_display_name)
+
+      raise UserNotFoundError if user.nil?
+
       @incoming_email.update_columns(user_id: user.id)
 
       body, @elided = select_body
@@ -195,13 +199,23 @@ module Email
     end
 
     def find_or_create_user(email, display_name)
-      username = UserNameSuggester.sanitize_username(display_name) if display_name.present?
+      user = nil
 
-      User.find_or_create_by(email: email) do |user|
-        user.username = UserNameSuggester.suggest(username.presence || email)
-        user.name = display_name.presence || User.suggest_name(email)
-        user.staged = true
+      User.transaction do
+        user = User.find_by_email(email)
+
+        if user.nil? && SiteSetting.enable_staged_users
+          username = UserNameSuggester.sanitize_username(display_name) if display_name.present?
+          user = User.create(
+            email: email,
+            username: UserNameSuggester.suggest(username.presence || email),
+            name: display_name.presence || User.suggest_name(email),
+            staged: true
+          )
+        end
       end
+
+      user
     end
 
     def destinations
@@ -382,7 +396,7 @@ module Email
               display_name = address_field.display_name.try(:to_s)
               if should_invite?(email)
                 user = find_or_create_user(email, display_name)
-                if can_invite?(topic, user)
+                if user && can_invite?(topic, user)
                   topic.topic_allowed_users.create!(user_id: user.id)
                   topic.add_small_action(sender, "invited_user", user.username)
                 end
diff --git a/spec/components/email/receiver_spec.rb b/spec/components/email/receiver_spec.rb
index 0dce786dc..01afbb6f8 100644
--- a/spec/components/email/receiver_spec.rb
+++ b/spec/components/email/receiver_spec.rb
@@ -21,6 +21,11 @@ describe Email::Receiver do
     expect { Email::Receiver.new("") }.to raise_error(Email::Receiver::EmptyEmailError)
   end
 
+  it "raises and UserNotFoundError when staged users are disabled" do
+    SiteSetting.enable_staged_users = false
+    expect { process(:user_not_found) }.to raise_error(Email::Receiver::UserNotFoundError)
+  end
+
   it "raises an AutoGeneratedEmailError when the mail is auto generated" do
     expect { process(:auto_generated_precedence) }.to raise_error(Email::Receiver::AutoGeneratedEmailError)
     expect { process(:auto_generated_header) }.to raise_error(Email::Receiver::AutoGeneratedEmailError)
diff --git a/spec/fixtures/emails/user_not_found.eml b/spec/fixtures/emails/user_not_found.eml
new file mode 100644
index 000000000..b2863e5d0
--- /dev/null
+++ b/spec/fixtures/emails/user_not_found.eml
@@ -0,0 +1,9 @@
+Return-Path: <user@not.found>
+From: Not Found <user@not.found>
+Date: Fri, 15 Jan 2016 00:12:43 +0100
+Message-ID: <50@foo.bar.mail>
+Mime-Version: 1.0
+Content-Type: text/plain
+Content-Transfer-Encoding: 7bit
+
+Email from an unknown user.