diff --git a/app/mailers/request_mailer.rb b/app/mailers/request_mailer.rb
index fa42e853f9..47daf34ee9 100644
--- a/app/mailers/request_mailer.rb
+++ b/app/mailers/request_mailer.rb
@@ -235,35 +235,47 @@ def requests_matching_email(email)
     InfoRequest.matching_incoming_email(addresses)
   end
 
+  def send_to_holding_pen(email, raw_email, opts)
+    opts[:rejected_reason] =
+      _("Could not identify the request from the email address")
+    request = InfoRequest.holding_pen_request
+    request.receive(email, raw_email, opts)
+  end
+
   # Member function, called on the new class made in self.receive above
   def receive(email, raw_email, source = :mailin)
     opts = { source: source }
 
-    # Find which info requests the email is for
-    reply_info_requests = requests_matching_email(email)
+    # Only check mail that doesn't have spam in the header
+    return if SpamAddress.spam?(MailHandler.get_all_addresses(email))
 
-    # Nothing found OR multiple different info requests, so save in holding pen
-    if reply_info_requests.empty? || reply_info_requests.count > 1
-      opts[:rejected_reason] =
-        _("Could not identify the request from the email address")
-      request = InfoRequest.holding_pen_request
+    # Find exact matches for info requests
+    exact_info_requests = requests_matching_email(email)
 
-      unless SpamAddress.spam?(MailHandler.get_all_addresses(email))
-        request.receive(email, raw_email, opts)
-      end
-      return
+    # Find any guesses for info requests
+    unless exact_info_requests.count == 1
+      guessed_info_requests = Guess.guessed_info_requests(email)
     end
 
-    # Send the message to each request, to be archived with it
-    reply_info_requests.each do |reply_info_request|
-      # If environment variable STOP_DUPLICATES is set, don't send message with same id again
-      if ENV['STOP_DUPLICATES']
-        if reply_info_request.already_received?(email, raw_email)
-          raise "message #{ email.message_id } already received by request"
-        end
+    # If there is only one info request matching mail, it gets attached to the
+    # request to be archived with it
+    if exact_info_requests.count == 1 || guessed_info_requests.count == 1
+      info_request = exact_info_requests.first || guessed_info_requests.first
+
+      if exact_info_requests.empty? && guessed_info_requests.count == 1
+        info_request.log_event(
+          'redeliver_incoming',
+          editor: 'automatic',
+          destination_request: info_request
+        )
       end
 
-      reply_info_request.receive(email, raw_email, opts)
+      info_request.receive(email, raw_email, opts)
+
+    else
+      # Otherwise, if there are no matching IRs, multiple IRs, or multiple IR
+      # guesses, we send the mail to the holding pen
+      send_to_holding_pen(email, raw_email, opts)
     end
   end
 
diff --git a/app/models/guess.rb b/app/models/guess.rb
new file mode 100644
index 0000000000..f75eafbf70
--- /dev/null
+++ b/app/models/guess.rb
@@ -0,0 +1,64 @@
+require 'text'
+
+##
+# A guess at which info request a incoming message should be associated to
+#
+class Guess
+  attr_reader :info_request, :components
+
+  # The percentage similarity the id or idhash much fulfil
+  THRESHOLD = 0.8
+
+  ##
+  # Return InfoRequest which we guess should receive an incoming message based
+  # on a threshold.
+  #
+  def self.guessed_info_requests(email)
+    # Match the email address in the message without matching the hash
+    email_addresses = MailHandler.get_all_addresses(email)
+    guesses = InfoRequest.guess_by_incoming_email(email_addresses)
+
+    guesses_reaching_threshold = guesses.select do |ir_guess|
+      id_score = ir_guess.id_score
+      idhash_score = ir_guess.idhash_score
+
+      (id_score == 1 && idhash_score >= THRESHOLD) ||
+        (id_score >= THRESHOLD && idhash_score == 1)
+    end
+
+    guesses_reaching_threshold.map(&:info_request).uniq
+  end
+
+  def initialize(info_request, **components)
+    @info_request = info_request
+    @components = components
+  end
+
+  def [](key)
+    components[key]
+  end
+
+  def id_score
+    return 1 unless self[:id]
+    similarity(self[:id], info_request.id)
+  end
+
+  def idhash_score
+    return 1 unless self[:idhash]
+    similarity(self[:idhash], info_request.idhash)
+  end
+
+  def ==(other)
+    info_request == other.info_request && components == other.components
+  end
+
+  def match_method
+    components.keys.first
+  end
+
+  private
+
+  def similarity(a, b)
+    Text::WhiteSimilarity.similarity(a.to_s, b.to_s)
+  end
+end
diff --git a/app/models/info_request.rb b/app/models/info_request.rb
index 2175f27dbe..4c1b25334c 100644
--- a/app/models/info_request.rb
+++ b/app/models/info_request.rb
@@ -40,7 +40,6 @@
 require 'fileutils'
 
 class InfoRequest < ApplicationRecord
-  Guess = Struct.new(:info_request, :matched_value, :match_method).freeze
   OLD_AGE_IN_DAYS = 21.days
 
   include Rails.application.routes.url_helpers
@@ -245,8 +244,13 @@ def self.guess_by_incoming_email(*emails)
     guesses = emails.flatten.reduce([]) do |memo, email|
       id, idhash = _extract_id_hash_from_email(email)
       id, idhash = _guess_idhash_from_email(email) if idhash.nil? || id.nil?
-      memo << Guess.new(find_by_id(id), email, :id)
-      memo << Guess.new(find_by_idhash(idhash), email, :idhash)
+
+      memo << Guess.new(
+        find_by_id(id), email: email, id: id, idhash: idhash
+      )
+      memo << Guess.new(
+        find_by_idhash(idhash), email: email, id: id, idhash: idhash
+      )
     end
 
     # Unique Guesses where we've found an `InfoRequest`
@@ -315,7 +319,7 @@ def self.guess_by_incoming_subject(subject_line)
       limit(25)
 
     guesses = requests.each.reduce([]) do |memo, request|
-      memo << Guess.new(request, subject_line, :subject)
+      memo << Guess.new(request, subject: subject_line)
     end
 
     # Unique Guesses where we've found an `InfoRequest`
diff --git a/app/views/admin_raw_email/_holding_pen.html.erb b/app/views/admin_raw_email/_holding_pen.html.erb
index 853c22c27e..fd5ec87113 100644
--- a/app/views/admin_raw_email/_holding_pen.html.erb
+++ b/app/views/admin_raw_email/_holding_pen.html.erb
@@ -54,12 +54,12 @@
               <code><%= guess.match_method %></code>
               <% if guess.match_method == :subject %>
                 because the incoming message has a subject of
-                <strong><%= guess.matched_value %></strong>
+                <strong><%= guess[:subject] %></strong>
               <% else %>
                 because it has an incoming email address
                 of <strong><%= guess.info_request.incoming_email %></strong>
                 and this incoming message was sent to
-              <strong><%= guess.matched_value %></strong>.
+              <strong><%= guess[:email] %></strong>.
               <% end %>
 
             </p>
diff --git a/spec/controllers/admin_raw_email_controller_spec.rb b/spec/controllers/admin_raw_email_controller_spec.rb
index ef923f2f42..3bf0025805 100644
--- a/spec/controllers/admin_raw_email_controller_spec.rb
+++ b/spec/controllers/admin_raw_email_controller_spec.rb
@@ -63,7 +63,10 @@
 
         it 'assigns guessed requests based on the hash' do
           get :show, params: { id: incoming_message.raw_email.id }
-          guess = InfoRequest::Guess.new(info_request, invalid_to, :idhash)
+          idhash = InfoRequest.hash_from_id(info_request.id)
+          guess = Guess.new(
+            info_request, email: invalid_to, id: nil, idhash: idhash
+          )
           expect(assigns[:guessed_info_requests]).to eq([guess])
         end
 
@@ -72,7 +75,7 @@
             FactoryBot.create(:incoming_message, subject: 'Basic Email').
               info_request
           get :show, params: { id: incoming_message.raw_email.id }
-          guess = InfoRequest::Guess.new(other_request, 'Basic Email', :subject)
+          guess = Guess.new(other_request, subject: 'Basic Email')
           expect(assigns[:guessed_info_requests]).to include(guess)
         end
 
diff --git a/spec/integration/admin_spec.rb b/spec/integration/admin_spec.rb
index e298236a4a..edfc4c0a96 100644
--- a/spec/integration/admin_spec.rb
+++ b/spec/integration/admin_spec.rb
@@ -127,26 +127,6 @@
         expect(page).to have_content "Only the authority can reply to this request"
       end
     end
-
-    it "guesses a misdirected request" do
-      info_request = FactoryBot.create(:info_request,
-                                       allow_new_responses_from: 'authority_only',
-                                       handle_rejected_responses: 'holding_pen')
-      mail_to = "request-#{info_request.id}-asdfg@example.com"
-      receive_incoming_mail('incoming-request-plain.email', email_to: mail_to)
-      interesting_email = last_holding_pen_mail
-
-      # now we add another message to the queue, which we're not interested in
-      receive_incoming_mail('incoming-request-plain.email',
-                            email_to: info_request.incoming_email,
-                            email_from: "")
-      expect(holding_pen_messages.length).to eq(2)
-      using_session(@admin) do
-        visit admin_raw_email_path interesting_email
-        expect(page).to have_content "Could not identify the request"
-        expect(page).to have_content info_request.title
-      end
-    end
   end
 
   describe 'generating an upload url' do
diff --git a/spec/mailers/request_mailer_spec.rb b/spec/mailers/request_mailer_spec.rb
index b2563bb73c..3cd72d6b3d 100644
--- a/spec/mailers/request_mailer_spec.rb
+++ b/spec/mailers/request_mailer_spec.rb
@@ -24,6 +24,25 @@
       deliveries.clear
     end
 
+    it "should append it to the appropriate request if there is only one guess of information request" do
+      ir = info_requests(:fancy_dog_request)
+      expect(ir.incoming_messages.count).to eq(1) # in the fixture
+      receive_incoming_mail(
+        'incoming-request-plain.email',
+        email_to: "request-#{ir.id}-#{ir.idhash}a@localhost"
+      )
+      expect(ir.incoming_messages.count).to eq(2) # one more arrives
+      expect(ir.info_request_events[-1].incoming_message_id).not_to be_nil
+      expect(ir.info_request_events[-2].params[:editor]).to eq("automatic")
+
+      deliveries = ActionMailer::Base.deliveries
+      expect(deliveries.size).to eq(1)
+      mail = deliveries[0]
+      # to the user who sent fancy_dog_request
+      expect(mail.to).to eq(['bob@localhost'])
+      deliveries.clear
+    end
+
     it "should store mail in holding pen and send to admin when the email is not to any information request" do
       ir = info_requests(:fancy_dog_request)
       expect(ir.incoming_messages.count).to eq(1)
diff --git a/spec/models/guess_spec.rb b/spec/models/guess_spec.rb
new file mode 100644
index 0000000000..d579bba0ae
--- /dev/null
+++ b/spec/models/guess_spec.rb
@@ -0,0 +1,95 @@
+require 'spec_helper'
+
+RSpec.describe Guess do
+  let(:info_request) do
+    FactoryBot.create(:info_request, id: 100, idhash: '4e637388')
+  end
+
+  describe '.guessed_info_requests' do
+    subject(:guesses) { described_class.guessed_info_requests(email) }
+
+    let(:email) do
+      mail = Mail.new
+      mail.to address
+      mail
+    end
+
+    let(:info_request) { FactoryBot.create(:info_request, id: 4566) }
+    let!(:other_info_request) { FactoryBot.create(:info_request) }
+
+    let(:id) { info_request.id }
+    let(:hash) { info_request.idhash }
+
+    context 'with email matching ID and ID hash' do
+      let(:address) { info_request.incoming_email }
+
+      it 'return matching InfoRequest' do
+        is_expected.to match_array([info_request])
+      end
+    end
+
+    context 'with email matching ID and almost ID hash' do
+      let(:address) { "request-#{id}-#{hash[0...-1]}}z@localhost" }
+
+      it 'return guessed InfoRequest' do
+        is_expected.to match_array([info_request])
+      end
+    end
+
+    context 'with email matching ID hash and almost ID' do
+      let(:address) { "request-#{id.to_s[0...-1]}-#{hash}@localhost" }
+
+      it 'return guessed InfoRequest' do
+        is_expected.to match_array([info_request])
+      end
+    end
+  end
+
+  describe 'with a subject line given' do
+    let(:guess) { described_class.new(info_request, subject: 'subject_line') }
+
+    it 'returns an id_score of 1' do
+      expect(guess.id_score).to eq(1)
+    end
+
+    it 'returns an idhash_score of 1' do
+      expect(guess.idhash_score).to eq(1)
+    end
+  end
+
+  describe 'with an id given' do
+    let(:guess_1) { described_class.new(info_request, id: 100) }
+    let(:guess_2) { described_class.new(info_request, id: 456) }
+    let(:guess_3) { described_class.new(info_request, id: 109) }
+
+    it 'returns an id_score of 1 when it is correct' do
+      expect(guess_1.id_score).to eq(1.0)
+    end
+
+    it 'returns an id_score of 0 when there is no similarity' do
+      expect(guess_2.id_score).to eq(0.0)
+    end
+
+    it 'returns a value between 0 and 1 when there is some similarity' do
+      expect(guess_3.id_score).to be_between(0, 1).exclusive
+    end
+  end
+
+  describe 'with an idhash given' do
+    let(:guess_1) { described_class.new(info_request, idhash: '4e637388') }
+    let(:guess_2) { described_class.new(info_request, idhash: '12345678') }
+    let(:guess_3) { described_class.new(info_request, idhash: '4e637399') }
+
+    it 'returns an idhash_score of 1 when it is correct' do
+      expect(guess_1.idhash_score).to eq(1.0)
+    end
+
+    it 'returns an idhash_score of 0 when there is no similarity' do
+      expect(guess_2.idhash_score).to eq(0.0)
+    end
+
+    it 'returns a value between 0 and 1 when there is some similarity' do
+      expect(guess_3.idhash_score).to be_between(0, 1).exclusive
+    end
+  end
+end
diff --git a/spec/models/info_request_spec.rb b/spec/models/info_request_spec.rb
index b5ab265ac7..6a0fd5920c 100644
--- a/spec/models/info_request_spec.rb
+++ b/spec/models/info_request_spec.rb
@@ -1910,7 +1910,16 @@
 
     context 'email with an intact id and broken idhash' do
       let(:email) { "request-#{ info_request.id }-asdfg@example.com" }
-      let(:guess) { described_class::Guess.new(info_request, email, :id) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: info_request.id,
+          idhash: 'asdfg'
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
@@ -1919,7 +1928,15 @@
         "request-#{ info_request.id }ab-#{ info_request.idhash }@example.com"
       end
 
-      let(:guess) { described_class::Guess.new(info_request, email, :idhash) }
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: nil,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
@@ -1928,13 +1945,30 @@
         "request-a12x3b-#{ info_request.idhash }@example.com"
       end
 
-      let(:guess) { described_class::Guess.new(info_request, email, :idhash) }
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: nil,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
     context 'upper case email with a broken id and otherwise intact idhash' do
       let(:email) { "REQUEST-123a-#{ info_request.idhash.upcase }@example.com" }
-      let(:guess) { described_class::Guess.new(info_request, email, :idhash) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: nil,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
@@ -1943,7 +1977,15 @@
         "request#{ info_request.id }#{ info_request.idhash }@example.com"
       end
 
-      let(:guess) { described_class::Guess.new(info_request, email, :id) }
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: info_request.id,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
@@ -1952,19 +1994,45 @@
         "request-#{ info_request.id }#{ info_request.idhash }@example.com"
       end
 
-      let(:guess) { described_class::Guess.new(info_request, email, :id) }
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: info_request.id,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
     context 'email with a broken id and an intact idhash but missing punctuation' do
       let(:email) { "request123ab#{ info_request.idhash }@example.com" }
-      let(:guess) { described_class::Guess.new(info_request, email, :idhash) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: nil,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
     context 'email with an intact id and a broken idhash but missing punctuation' do
       let(:email) { "request#{ info_request.id }abcdefgh@example.com" }
-      let(:guess) { described_class::Guess.new(info_request, email, :id) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: info_request.id,
+          idhash: 'abcdefgh'
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
@@ -1972,13 +2040,31 @@
       before { InfoRequest.where(id: 1_231_014).destroy_all }
       let!(:info_request) { FactoryBot.create(:info_request, id: 1_231_014) }
       let(:email) { 'request-123loL4abcdefgh@example.com' }
-      let(:guess) { described_class::Guess.new(info_request, email, :id) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: info_request.id,
+          idhash: 'abcdefgh'
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
     context 'email with a broken id and an intact idhash but broken format' do
       let(:email) { "reqeust=123ab#{ info_request.idhash }@example.com" }
-      let(:guess) { described_class::Guess.new(info_request, email, :idhash) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email,
+          id: nil,
+          idhash: info_request.idhash
+        )
+      end
+
       it { is_expected.to include(guess) }
     end
 
@@ -2000,10 +2086,22 @@
         "request-#{ info_request_1.id }-#{ info_request_2.idhash }@example.com"
       end
 
-      let(:guess_1) { described_class::Guess.new(info_request_1, email, :id) }
+      let(:guess_1) do
+        Guess.new(
+          info_request_1,
+          email: email,
+          id: info_request_1.id,
+          idhash: info_request_2.idhash
+        )
+      end
 
       let(:guess_2) do
-        described_class::Guess.new(info_request_2, email, :idhash)
+        Guess.new(
+          info_request_2,
+          email: email,
+          id: info_request_1.id,
+          idhash: info_request_2.idhash
+        )
       end
 
       it { is_expected.to match_array([guess_1, guess_2]) }
@@ -2013,7 +2111,15 @@
       let(:email_1) { "request-123ab-#{ info_request.idhash }@example.com" }
       let(:email_2) { "request-#{ info_request.id }-asdfg@example.com" }
       let(:email) { [email_1, email_2] }
-      let(:guess) { described_class::Guess.new(info_request, email_1, :idhash) }
+
+      let(:guess) do
+        Guess.new(
+          info_request,
+          email: email_1,
+          id: nil,
+          idhash: info_request.idhash
+        )
+      end
 
       it { is_expected.to match_array([guess]) }
     end
@@ -2032,10 +2138,22 @@
 
       let(:email) { [email_1, email_2] }
 
-      let(:guess_1) { described_class::Guess.new(info_request_1, email_1, :id) }
+      let(:guess_1) do
+        Guess.new(
+          info_request_1,
+          email: email_1,
+          id: info_request_1.id,
+          idhash: info_request_2.idhash
+        )
+      end
 
       let(:guess_2) do
-        described_class::Guess.new(info_request_2, email_1, :idhash)
+        Guess.new(
+          info_request_2,
+          email: email_1,
+          id: info_request_1.id,
+          idhash: info_request_2.idhash
+        )
       end
 
       it { is_expected.to match_array([guess_1, guess_2]) }
@@ -2055,9 +2173,7 @@
     context 'a direct reply to the original request email' do
       let(:subject_line) { info_request.email_subject_followup }
 
-      let(:guess) do
-        described_class::Guess.new(info_request, subject_line, :subject)
-      end
+      let(:guess) { Guess.new(info_request, subject: subject_line) }
 
       it { is_expected.to include(guess) }
     end
@@ -2067,9 +2183,7 @@
         info_request.email_subject_followup.gsub('Re: ', 'RE: ')
       end
 
-      let(:guess) do
-        described_class::Guess.new(info_request, subject_line, :subject)
-      end
+      let(:guess) { Guess.new(info_request, subject: subject_line) }
 
       it { is_expected.to include(guess) }
     end
@@ -2087,13 +2201,8 @@
         'Re: Freedom of Information request - How many jelly beans?'
       end
 
-      let(:guess_1) do
-        described_class::Guess.new(info_request_1, subject_line, :subject)
-      end
-
-      let(:guess_2) do
-        described_class::Guess.new(info_request_2, subject_line, :subject)
-      end
+      let(:guess_1) { Guess.new(info_request_1, subject: subject_line) }
+      let(:guess_2) { Guess.new(info_request_2, subject: subject_line) }
 
       it { is_expected.to match_array([guess_1, guess_2]) }
     end
@@ -2114,10 +2223,7 @@
       end
 
       let(:subject_line) { 'Our ref: 12345678' }
-
-      let(:guess) do
-        described_class::Guess.new(info_request, subject_line, :subject)
-      end
+      let(:guess) { Guess.new(info_request, subject: subject_line) }
 
       it { is_expected.to include(guess) }
     end
@@ -2132,9 +2238,7 @@
       end
 
       let(:guess) do
-        described_class::Guess.new(InfoRequest.holding_pen_request,
-                                   subject_line,
-                                   :subject)
+        Guess.new(InfoRequest.holding_pen_request, subject: subject_line)
       end
 
       it { is_expected.to_not include(guess) }
@@ -2148,9 +2252,7 @@
                                              info_request: info_request)
       end
 
-      let(:guess) do
-        described_class::Guess.new(info_request, subject_line, :subject)
-      end
+      let(:guess) { Guess.new(info_request, subject: subject_line) }
 
       it { is_expected.to include(guess) }
     end
@@ -2166,13 +2268,8 @@
         FactoryBot.create(:incoming_message, subject: subject_line).info_request
       end
 
-      let(:guess_1) do
-        described_class::Guess.new(info_request_1, subject_line, :subject)
-      end
-
-      let(:guess_2) do
-        described_class::Guess.new(info_request_2, subject_line, :subject)
-      end
+      let(:guess_1) { Guess.new(info_request_1, subject: subject_line) }
+      let(:guess_2) { Guess.new(info_request_2, subject: subject_line) }
 
       it { is_expected.to match_array([guess_1, guess_2]) }
     end