FIX: Add random suffix to outbound Message-ID for email (#15179)

Currently the Message-IDs we send out for outbound email
are not unique; for a post they look like:

topic/TOPIC_ID/POST_ID@HOST

And for a topic they look like:

topic/TOPIC_ID@HOST

This commit changes the outbound Message-IDs to also have
a random suffix before the host, so the new format is
like this:

topic/TOPIC_ID/POST_ID.RANDOM_SUFFIX@HOST

Or:

topic/TOPIC_ID.RANDOM_SUFFIX@HOST

This should help with email deliverability. This change
is backwards-compatible, the old Message-ID format will
still be recognized in the mail receiver flow, so people
will still be able to reply using Message-IDs, In-Reply-To,
and References headers that have already been sent.

This commit also refactors Message-ID related logic
to a central location, and adds judicious amounts of
tests and documentation.

app/controllers/webhooks_controller.rb

@@ -13,7 +13,7 @@ class WebhooksController < ActionController::Base
   def sendgrid
     events = params["_json"] || [params]
     events.each do |event|
-      message_id = Email.message_id_clean((event["smtp-id"] || ""))
+      message_id = Email::MessageIdService.message_id_clean((event["smtp-id"] || ""))
       to_address = event["email"]
       if event["event"] == "bounce"
         if event["status"]["4."]
@@ -150,7 +150,7 @@ class WebhooksController < ActionController::Base
     return mailgun_failure unless valid_mailgun_signature?(params["token"], params["timestamp"], params["signature"])
 
     event = params["event"]
-    message_id = Email.message_id_clean(params["Message-Id"])
+    message_id = Email::MessageIdService.message_id_clean(params["Message-Id"])
     to_address = params["recipient"]
 
     # only handle soft bounces, because hard bounces are also handled

lib/email.rb

@@ -52,21 +52,8 @@ module Email
     SiteSetting.email_site_title.presence || SiteSetting.title
   end
 
-  # https://tools.ietf.org/html/rfc850#section-2.1.7
-  def self.message_id_rfc_format(message_id)
-    message_id.present? && !is_message_id_rfc?(message_id) ? "<#{message_id}>" : message_id
-  end
-
-  def self.message_id_clean(message_id)
-    message_id.present? && is_message_id_rfc?(message_id) ? message_id.gsub(/^<|>$/, "") : message_id
-  end
-
   private
 
-  def self.is_message_id_rfc?(message_id)
-    message_id.start_with?('<') && message_id.include?('@') && message_id.end_with?('>')
-  end
-
   def self.obfuscate_part(part)
     if part.size < 3
       "*" * part.size

lib/email/message_id_service.rb

@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+module Email
+  ##
+  # Email Message-IDs are used in both our outbound and inbound email
+  # flow. For the outbound flow via Email::Sender, we assign a unique
+  # Message-ID for any emails sent out from the application.
+  # If we are sending an email related to a topic, such as through the
+  # PostAlerter class, then the Message-ID will contain references to
+  # the topic ID, and if it is for a specific post, the post ID,
+  # along with a random suffix to make the Message-ID truly unique.
+  # The host must also be included on the Message-IDs.
+  #
+  # For the inbound email flow via Email::Receiver, we use Message-IDs
+  # to discern which topic or post the inbound email reply should be
+  # in response to. In this case, the Message-ID is extracted from the
+  # References and/or In-Reply-To headers, and compared with either
+  # the IncomingEmail table, the Post table, or the IncomingEmail to
+  # determine where to send the reply.
+  #
+  # See https://datatracker.ietf.org/doc/html/rfc2822#section-3.6.4 for
+  # more specific information around Message-IDs in email.
+  #
+  # See https://tools.ietf.org/html/rfc850#section-2.1.7 for the
+  # Message-ID format specification.
+  class MessageIdService
+    class << self
+      def generate_default
+        "<#{SecureRandom.uuid}@#{host}>"
+      end
+
+      def generate_for_post(post, use_incoming_email_if_present: false)
+        if use_incoming_email_if_present && post.incoming_email&.message_id.present?
+          return "<#{post.incoming_email.message_id}>"
+        end
+
+        "<topic/#{post.topic_id}/#{post.id}.#{random_suffix}@#{host}>"
+      end
+
+      def generate_for_topic(topic, use_incoming_email_if_present: false)
+        first_post = topic.ordered_posts.first
+
+        if use_incoming_email_if_present && first_post.incoming_email&.message_id.present?
+          return "<#{first_post.incoming_email.message_id}>"
+        end
+
+        "<topic/#{topic.id}.#{random_suffix}@#{host}>"
+      end
+
+      def find_post_from_message_ids(message_ids)
+        message_ids = message_ids.map { |message_id| message_id_clean(message_id) }
+        post_ids =  message_ids.map { |message_id| message_id[message_id_post_id_regexp, 1] }.compact.map(&:to_i)
+        post_ids << Post.where(
+          topic_id: message_ids.map { |message_id| message_id[message_id_topic_id_regexp, 1] }.compact,
+          post_number: 1
+        ).pluck(:id)
+        post_ids << EmailLog.where(message_id: message_ids).pluck(:post_id)
+        post_ids << IncomingEmail.where(message_id: message_ids).pluck(:post_id)
+
+        post_ids.flatten!
+        post_ids.compact!
+        post_ids.uniq!
+
+        return if post_ids.empty?
+
+        Post.where(id: post_ids).order(:created_at).last
+      end
+
+      def random_suffix
+        SecureRandom.hex(12)
+      end
+
+      def discourse_generated_message_id?(message_id)
+        !!(message_id =~ message_id_post_id_regexp) ||
+          !!(message_id =~ message_id_topic_id_regexp)
+      end
+
+      def message_id_post_id_regexp
+        @message_id_post_id_regexp ||= Regexp.new "topic/\\d+/(\\d+|\\d+\.\\w+)@#{Regexp.escape(host)}"
+      end
+
+      def message_id_topic_id_regexp
+        @message_id_topic_id_regexp ||= Regexp.new "topic/(\\d+|\\d+\.\\w+)@#{Regexp.escape(host)}"
+      end
+
+      def message_id_rfc_format(message_id)
+        message_id.present? && !is_message_id_rfc?(message_id) ? "<#{message_id}>" : message_id
+      end
+
+      def message_id_clean(message_id)
+        message_id.present? && is_message_id_rfc?(message_id) ? message_id.gsub(/^<|>$/, "") : message_id
+      end
+
+      def is_message_id_rfc?(message_id)
+        message_id.start_with?('<') && message_id.include?('@') && message_id.end_with?('>')
+      end
+
+      def host
+        @host ||= Email::Sender.host_for(Discourse.base_url)
+      end
+    end
+  end
+end

lib/email/receiver.rb

@@ -107,7 +107,7 @@ module Email
       # server (e.g. a message_id generated by Gmail) and does not need to
       # be updated, because message_ids from the IMAP server are not guaranteed
       # to be unique.
-      return unless discourse_generated_message_id?(@message_id)
+      return unless Email::MessageIdService.discourse_generated_message_id?(@message_id)
 
       incoming_email.update(
         imap_uid_validity: @opts[:imap_uid_validity],
@@ -801,7 +801,7 @@ module Email
       # if the user is directly replying to an email send to them from discourse,
       # there will be a corresponding EmailLog record, so we can use that as the
       # reply post if it exists
-      if discourse_generated_message_id?(mail.in_reply_to)
+      if Email::MessageIdService.discourse_generated_message_id?(mail.in_reply_to)
         post_id_from_email_log = EmailLog.where(message_id: mail.in_reply_to)
           .addressed_to_user(user)
           .order(created_at: :desc)
@@ -1056,35 +1056,7 @@ module Email
       message_ids = Email::Receiver.extract_reply_message_ids(@mail, max_message_id_count: 5)
       return if message_ids.empty?
 
-      post_ids =  message_ids.map { |message_id| message_id[message_id_post_id_regexp, 1] }.compact.map(&:to_i)
+      Email::MessageIdService.find_post_from_message_ids(message_ids)
-      post_ids << Post.where(topic_id: message_ids.map { |message_id| message_id[message_id_topic_id_regexp, 1] }.compact, post_number: 1).pluck(:id)
-      post_ids << EmailLog.where(message_id: message_ids).pluck(:post_id)
-      post_ids << IncomingEmail.where(message_id: message_ids).pluck(:post_id)
-
-      post_ids.flatten!
-      post_ids.compact!
-      post_ids.uniq!
-
-      return if post_ids.empty?
-
-      Post.where(id: post_ids).order(:created_at).last
-    end
-
-    def host
-      @host ||= Email::Sender.host_for(Discourse.base_url)
-    end
-
-    def discourse_generated_message_id?(message_id)
-      !!(message_id =~ message_id_post_id_regexp) ||
-        !!(message_id =~ message_id_topic_id_regexp)
-    end
-
-    def message_id_post_id_regexp
-      @message_id_post_id_regexp ||= Regexp.new "topic/\\d+/(\\d+)@#{Regexp.escape(host)}"
-    end
-
-    def message_id_topic_id_regexp
-      @message_id_topic_id_regexp ||= Regexp.new "topic/(\\d+)@#{Regexp.escape(host)}"
     end
 
     def self.extract_reply_message_ids(mail, max_message_id_count:)
@@ -1100,7 +1072,7 @@ module Email
         references
       elsif references.present?
         references.split(/[\s,]/).map do |r|
-          Email.message_id_clean(r)
+          Email::MessageIdService.message_id_clean(r)
         end
       end
     end

lib/email/sender.rb

@@ -109,7 +109,7 @@ module Email
       ).pluck_first(:id)
 
       # always set a default Message ID from the host
-      @message.header['Message-ID'] = "<#{SecureRandom.uuid}@#{host}>"
+      @message.header['Message-ID'] = Email::MessageIdService.generate_default
 
       if topic_id.present? && post_id.present?
         post = Post.find_by(id: post_id, topic_id: topic_id)
@@ -121,15 +121,9 @@ module Email
         return skip(SkippedEmailLog.reason_types[:sender_topic_deleted]) if topic.blank?
 
         add_attachments(post)
-        first_post = topic.ordered_posts.first
 
-        topic_message_id = first_post.incoming_email&.message_id.present? ?
+        topic_message_id = Email::MessageIdService.generate_for_topic(topic, use_incoming_email_if_present: true)
-          "<#{first_post.incoming_email.message_id}>" :
+        post_message_id = Email::MessageIdService.generate_for_post(post, use_incoming_email_if_present: true)
-          "<topic/#{topic_id}@#{host}>"
-
-        post_message_id = post.incoming_email&.message_id.present? ?
-          "<#{post.incoming_email.message_id}>" :
-          "<topic/#{topic_id}/#{post_id}@#{host}>"
 
         referenced_posts = Post.includes(:incoming_email)
           .joins("INNER JOIN post_replies ON post_replies.post_id = posts.id ")
@@ -141,9 +135,9 @@ module Email
             "<#{referenced_post.incoming_email.message_id}>"
           else
             if referenced_post.post_number == 1
-              "<topic/#{topic_id}@#{host}>"
+              Email::MessageIdService.generate_for_topic(topic)
             else
-              "<topic/#{topic_id}/#{referenced_post.id}@#{host}>"
+              Email::MessageIdService.generate_for_post(referenced_post)
             end
           end
         end

lib/imap/providers/generic.rb

@@ -236,7 +236,7 @@ module Imap
           trashed_email_uids = find_uids_by_message_ids(message_ids)
           if trashed_email_uids.any?
             trashed_emails = emails(trashed_email_uids, ["UID", "ENVELOPE"]).map do |e|
-              BasicMail.new(message_id: Email.message_id_clean(e['ENVELOPE'].message_id), uid: e['UID'])
+              BasicMail.new(message_id: Email::MessageIdService.message_id_clean(e['ENVELOPE'].message_id), uid: e['UID'])
             end
           end
         end
@@ -253,7 +253,7 @@ module Imap
           spam_email_uids = find_uids_by_message_ids(message_ids)
           if spam_email_uids.any?
             spam_emails = emails(spam_email_uids, ["UID", "ENVELOPE"]).map do |e|
-              BasicMail.new(message_id: Email.message_id_clean(e['ENVELOPE'].message_id), uid: e['UID'])
+              BasicMail.new(message_id: Email::MessageIdService.message_id_clean(e['ENVELOPE'].message_id), uid: e['UID'])
             end
           end
         end
@@ -266,7 +266,7 @@ module Imap
 
       def find_uids_by_message_ids(message_ids)
         header_message_id_terms = message_ids.map do |msgid|
-          "HEADER Message-ID '#{Email.message_id_rfc_format(msgid)}'"
+          "HEADER Message-ID '#{Email::MessageIdService.message_id_rfc_format(msgid)}'"
         end
 
         # OR clauses are written in Polish notation...so the query looks like this:

lib/imap/sync.rb

@@ -138,7 +138,7 @@ module Imap
         else
           # try finding email by message-id instead, we may be able to set the uid etc.
           incoming_email = IncomingEmail.where(
-            message_id: Email.message_id_clean(email['ENVELOPE'].message_id),
+            message_id: Email::MessageIdService.message_id_clean(email['ENVELOPE'].message_id),
             imap_uid: nil,
             imap_uid_validity: nil
           ).where("to_addresses LIKE ?", "%#{@group.email_username}%").first

spec/components/email/email_spec.rb

@@ -64,30 +64,4 @@ describe Email do
     end
 
   end
-
-  describe "message_id_rfc_format" do
-
-    it "returns message ID in RFC format" do
-      expect(Email.message_id_rfc_format("test@test")).to eq("<test@test>")
-    end
-
-    it "returns input if already in RFC format" do
-      expect(Email.message_id_rfc_format("<test@test>")).to eq("<test@test>")
-    end
-
-  end
-
-  describe "message_id_clean" do
-
-    it "returns message ID if in RFC format" do
-      expect(Email.message_id_clean("<test@test>")).to eq("test@test")
-    end
-
-    it "returns input if a clean message ID is not in RFC format" do
-      message_id = "<" + "@" * 50
-      expect(Email.message_id_clean(message_id)).to eq(message_id)
-    end
-
-  end
-
 end

spec/components/email/receiver_spec.rb

@@ -947,6 +947,52 @@ describe Email::Receiver do
         ordered_posts[1..-1].each(&:trash!)
         expect { process(:email_reply_4) }.to change { topic.posts.count }.by(1)
       end
+
+      describe "replying with various message-id formats" do
+        let!(:topic) do
+          process(:email_reply_1)
+          Topic.last
+        end
+        let!(:post) { Fabricate(:post, topic: topic) }
+
+        def process_mail_with_message_id(message_id)
+          mail_string = <<~REPLY
+          Return-Path: <two@foo.com>
+          From: Two <two@foo.com>
+          To: one@foo.com
+          Subject: RE: Testing email threading
+          Date: Fri, 15 Jan 2016 00:12:43 +0100
+          Message-ID: <44@foo.bar.mail>
+          In-Reply-To: <#{message_id}>
+          Mime-Version: 1.0
+          Content-Type: text/plain
+          Content-Transfer-Encoding: 7bit
+
+          This is email reply testing with Message-ID formats.
+          REPLY
+          Email::Receiver.new(mail_string).process!
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID/POST_ID@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}/#{post.id}@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID/POST_ID.RANDOM_SUFFIX@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}/#{post.id}.rjc3yr79834y@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+
+        it "posts a reply using a message-id in the format topic/TOPIC_ID.RANDOM_SUFFIX@HOST" do
+          expect { process_mail_with_message_id("topic/#{topic.id}/#{post.id}.x3487nxy877843x@test.localhost") }.to change { Post.count }.by(1)
+          expect(topic.reload.posts.last.raw).to include("This is email reply testing with Message-ID formats")
+        end
+      end
     end
 
     it "supports any kind of attachments when 'allow_all_attachments_for_group_messages' is enabled" do
@@ -1161,6 +1207,7 @@ describe Email::Receiver do
         NotificationEmailer.enable
         SiteSetting.disallow_reply_by_email_after_days = 10000
         Jobs.run_immediately!
+        Email::MessageIdService.stubs(:random_suffix).returns("blah123")
       end
 
       def reply_as_group_user
@@ -1185,7 +1232,7 @@ describe Email::Receiver do
 
       it "creates an EmailLog when someone from the group replies, and does not create an IncomingEmail record for the reply" do
         email_log, group_post = reply_as_group_user
-        expect(email_log.message_id).to eq("topic/#{original_inbound_email_topic.id}/#{group_post.id}@test.localhost")
+        expect(email_log.message_id).to eq("topic/#{original_inbound_email_topic.id}/#{group_post.id}.blah123@test.localhost")
         expect(email_log.to_address).to eq("two@foo.com")
         expect(email_log.email_type).to eq("user_private_message")
         expect(email_log.post_id).to eq(group_post.id)

spec/components/email/sender_spec.rb

@@ -260,6 +260,7 @@ describe Email::Sender do
     end
 
     context "email threading" do
+      let(:random_message_id_suffix) { "5f1330cfd941f323d7f99b9e" }
       fab!(:topic) { Fabricate(:topic) }
 
       fab!(:post_1) { Fabricate(:post, topic: topic, post_number: 1) }
@@ -271,14 +272,17 @@ describe Email::Sender do
       let!(:post_reply_2_4) { PostReply.create(post: post_2, reply: post_4) }
       let!(:post_reply_3_4) { PostReply.create(post: post_3, reply: post_4) }
 
-      before { message.header['X-Discourse-Topic-Id'] = topic.id }
+      before do
+        message.header['X-Discourse-Topic-Id'] = topic.id
+        Email::MessageIdService.stubs(:random_suffix).returns(random_message_id_suffix)
+      end
 
       it "doesn't set the 'In-Reply-To' and 'References' headers on the first post" do
         message.header['X-Discourse-Post-Id'] = post_1.id
 
         email_sender.send
 
-        expect(message.header['Message-Id'].to_s).to eq("<topic/#{topic.id}@test.localhost>")
+        expect(message.header['Message-Id'].to_s).to eq("<topic/#{topic.id}.#{random_message_id_suffix}@test.localhost>")
         expect(message.header['In-Reply-To'].to_s).to be_blank
         expect(message.header['References'].to_s).to be_blank
       end
@@ -288,8 +292,8 @@ describe Email::Sender do
 
         email_sender.send
 
-        expect(message.header['Message-Id'].to_s).to eq("<topic/#{topic.id}/#{post_2.id}@test.localhost>")
+        expect(message.header['Message-Id'].to_s).to eq("<topic/#{topic.id}/#{post_2.id}.#{random_message_id_suffix}@test.localhost>")
-        expect(message.header['In-Reply-To'].to_s).to eq("<topic/#{topic.id}@test.localhost>")
+        expect(message.header['In-Reply-To'].to_s).to eq("<topic/#{topic.id}.#{random_message_id_suffix}@test.localhost>")
       end
 
       it "sets the 'In-Reply-To' header to the newest replied post" do
@@ -297,8 +301,8 @@ describe Email::Sender do
 
         email_sender.send
 
-        expect(message.header['Message-Id'].to_s).to eq("<topic/#{topic.id}/#{post_4.id}@test.localhost>")
+        expect(message.header['Message-Id'].to_s).to eq("<topic/#{topic.id}/#{post_4.id}.#{random_message_id_suffix}@test.localhost>")
-        expect(message.header['In-Reply-To'].to_s).to eq("<topic/#{topic.id}/#{post_3.id}@test.localhost>")
+        expect(message.header['In-Reply-To'].to_s).to eq("<topic/#{topic.id}/#{post_3.id}.#{random_message_id_suffix}@test.localhost>")
       end
 
       it "sets the 'References' header to the topic and all replied posts" do
@@ -307,9 +311,9 @@ describe Email::Sender do
         email_sender.send
 
         references = [
-          "<topic/#{topic.id}@test.localhost>",
+          "<topic/#{topic.id}.#{random_message_id_suffix}@test.localhost>",
-          "<topic/#{topic.id}/#{post_3.id}@test.localhost>",
+          "<topic/#{topic.id}/#{post_3.id}.#{random_message_id_suffix}@test.localhost>",
-          "<topic/#{topic.id}/#{post_2.id}@test.localhost>",
+          "<topic/#{topic.id}/#{post_2.id}.#{random_message_id_suffix}@test.localhost>",
         ]
 
         expect(message.header['References'].to_s).to eq(references.join(" "))
@@ -328,7 +332,7 @@ describe Email::Sender do
 
         references = [
           "<#{topic_incoming_email.message_id}>",
-          "<topic/#{topic.id}/#{post_3.id}@test.localhost>",
+          "<topic/#{topic.id}/#{post_3.id}.#{random_message_id_suffix}@test.localhost>",
           "<#{post_2_incoming_email.message_id}>",
         ]
 

spec/jobs/regular/group_smtp_email_spec.rb

@@ -23,6 +23,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
   let(:staged1) { Fabricate(:staged, email: "otherguy@test.com") }
   let(:staged2) { Fabricate(:staged, email: "cormac@lit.com") }
   let(:normaluser) { Fabricate(:user, email: "justanormalguy@test.com", username: "normaluser") }
+  let(:random_message_id_suffix) { "5f1330cfd941f323d7f99b9e" }
 
   before do
     SiteSetting.enable_smtp = true
@@ -34,6 +35,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
     TopicAllowedUser.create(user: staged1, topic: topic)
     TopicAllowedUser.create(user: staged2, topic: topic)
     TopicAllowedUser.create(user: normaluser, topic: topic)
+    Email::MessageIdService.stubs(:random_suffix).returns(random_message_id_suffix)
   end
 
   it "sends an email using the GroupSmtpMailer and Email::Sender" do
@@ -61,7 +63,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
     PostReply.create(post: second_post, reply: post)
     subject.execute(args)
     email_log = EmailLog.find_by(post_id: post.id, topic_id: post.topic_id, user_id: recipient_user.id)
-    expect(email_log.raw_headers).to include("In-Reply-To: <topic/#{post.topic_id}/#{second_post.id}@#{Email::Sender.host_for(Discourse.base_url)}>")
+    expect(email_log.raw_headers).to include("In-Reply-To: <topic/#{post.topic_id}/#{second_post.id}.#{random_message_id_suffix}@#{Email::Sender.host_for(Discourse.base_url)}>")
     expect(email_log.as_mail_message.html_part.to_s).not_to include(I18n.t("user_notifications.in_reply_to"))
   end
 
@@ -82,7 +84,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
     subject.execute(args)
     email_log = EmailLog.find_by(post_id: post.id, topic_id: post.topic_id, user_id: recipient_user.id)
     expect(email_log).not_to eq(nil)
-    expect(email_log.message_id).to eq("topic/#{post.topic_id}/#{post.id}@test.localhost")
+    expect(email_log.message_id).to eq("topic/#{post.topic_id}/#{post.id}.#{random_message_id_suffix}@test.localhost")
   end
 
   it "creates an IncomingEmail record with the correct details to avoid double processing IMAP" do
@@ -91,7 +93,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
     expect(ActionMailer::Base.deliveries.last.subject).to eq("Re: Help I need support")
     incoming_email = IncomingEmail.find_by(post_id: post.id, topic_id: post.topic_id, user_id: post.user.id)
     expect(incoming_email).not_to eq(nil)
-    expect(incoming_email.message_id).to eq("topic/#{post.topic_id}/#{post.id}@test.localhost")
+    expect(incoming_email.message_id).to eq("topic/#{post.topic_id}/#{post.id}.#{random_message_id_suffix}@test.localhost")
     expect(incoming_email.created_via).to eq(IncomingEmail.created_via_types[:group_smtp])
     expect(incoming_email.to_addresses).to eq("test@test.com")
     expect(incoming_email.cc_addresses).to eq("otherguy@test.com;cormac@lit.com")
@@ -115,7 +117,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
     expect(ActionMailer::Base.deliveries.last.subject).to eq("Re: Help I need support")
     email_log = EmailLog.find_by(post_id: post.id, topic_id: post.topic_id, user_id: recipient_user.id)
     expect(email_log).not_to eq(nil)
-    expect(email_log.message_id).to eq("topic/#{post.topic_id}/#{post.id}@test.localhost")
+    expect(email_log.message_id).to eq("topic/#{post.topic_id}/#{post.id}.#{random_message_id_suffix}@test.localhost")
   end
 
   it "creates an IncomingEmail record with the correct details to avoid double processing IMAP" do
@@ -124,7 +126,7 @@ RSpec.describe Jobs::GroupSmtpEmail do
     expect(ActionMailer::Base.deliveries.last.subject).to eq("Re: Help I need support")
     incoming_email = IncomingEmail.find_by(post_id: post.id, topic_id: post.topic_id, user_id: post.user.id)
     expect(incoming_email).not_to eq(nil)
-    expect(incoming_email.message_id).to eq("topic/#{post.topic_id}/#{post.id}@test.localhost")
+    expect(incoming_email.message_id).to eq("topic/#{post.topic_id}/#{post.id}.#{random_message_id_suffix}@test.localhost")
     expect(incoming_email.created_via).to eq(IncomingEmail.created_via_types[:group_smtp])
     expect(incoming_email.to_addresses).to eq("test@test.com")
     expect(incoming_email.cc_addresses).to eq("otherguy@test.com;cormac@lit.com")

spec/lib/message_id_service_spec.rb

@@ -0,0 +1,117 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe Email::MessageIdService do
+  fab!(:topic) { Fabricate(:topic) }
+  fab!(:post) { Fabricate(:post, topic: topic) }
+  fab!(:second_post) { Fabricate(:post, topic: topic) }
+
+  subject { described_class }
+
+  describe "#generate_for_post" do
+    it "generates for the post using the message_id on the post's incoming_email" do
+      Fabricate(:incoming_email, message_id: "test@test.localhost", post: post)
+      post.reload
+      expect(subject.generate_for_post(post, use_incoming_email_if_present: true)).to eq("<test@test.localhost>")
+    end
+
+    it "generates for the post without an incoming_email record" do
+      expect(subject.generate_for_post(post)).to match(subject.message_id_post_id_regexp)
+      expect(subject.generate_for_post(post, use_incoming_email_if_present: true)).to match(subject.message_id_post_id_regexp)
+    end
+  end
+
+  describe "#generate_for_topic" do
+    it "generates for the topic using the message_id on the first post's incoming_email" do
+      Fabricate(:incoming_email, message_id: "test@test.localhost", post: post)
+      post.reload
+      expect(subject.generate_for_topic(topic, use_incoming_email_if_present: true)).to eq("<test@test.localhost>")
+    end
+
+    it "generates for the topic without an incoming_email record" do
+      expect(subject.generate_for_topic(topic)).to match(subject.message_id_topic_id_regexp)
+      expect(subject.generate_for_topic(topic, use_incoming_email_if_present: true)).to match(subject.message_id_topic_id_regexp)
+    end
+  end
+
+  describe "find_post_from_message_ids" do
+    let(:post_format_message_id) { "<topic/#{topic.id}/#{post.id}.test123@test.localhost>" }
+    let(:topic_format_message_id) { "<topic/#{topic.id}.test123@test.localhost>" }
+    let(:default_format_message_id) { "<36ac1ddd-5083-461d-b72c-6372fb0e7f33@test.localhost>" }
+    let(:gmail_format_message_id) { "<CAPGrNgZ7QEFuPcsxJBRZLhBhAYPO_ruYpCANSdqiQEbc9Otpiw@mail.gmail.com>" }
+
+    it "finds a post based only on a post-format message id" do
+      expect(subject.find_post_from_message_ids([post_format_message_id])).to eq(post)
+    end
+
+    it "finds a post based only on a topic-format message id" do
+      expect(subject.find_post_from_message_ids([topic_format_message_id])).to eq(post)
+    end
+
+    it "finds a post from the email log" do
+      email_log = Fabricate(:email_log, message_id: subject.message_id_clean(default_format_message_id))
+      expect(subject.find_post_from_message_ids([default_format_message_id])).to eq(email_log.post)
+    end
+
+    it "finds a post from the incoming email log" do
+      incoming_email = Fabricate(
+        :incoming_email,
+        message_id: subject.message_id_clean(gmail_format_message_id),
+        post: Fabricate(:post)
+      )
+      expect(subject.find_post_from_message_ids([gmail_format_message_id])).to eq(incoming_email.post)
+    end
+
+    it "gets the last created post if multiple are returned" do
+      incoming_email = Fabricate(
+        :incoming_email,
+        message_id: subject.message_id_clean(post_format_message_id),
+        post: Fabricate(:post, created_at: 10.days.ago)
+      )
+      expect(subject.find_post_from_message_ids([post_format_message_id])).to eq(post)
+    end
+  end
+
+  describe "#discourse_generated_message_id?" do
+    def check_format(message_id)
+      subject.discourse_generated_message_id?(message_id)
+    end
+
+    it "works correctly for the different possible formats" do
+      expect(check_format("topic/1223/4525.3c4f8n9@test.localhost")).to eq(true)
+      expect(check_format("<topic/1223/4525.3c4f8n9@test.localhost>")).to eq(true)
+      expect(check_format("topic/1223.fc3j4843@test.localhost")).to eq(true)
+      expect(check_format("<topic/1223.fc3j4843@test.localhost>")).to eq(true)
+      expect(check_format("topic/1223/4525@test.localhost")).to eq(true)
+      expect(check_format("<topic/1223/4525@test.localhost>")).to eq(true)
+      expect(check_format("topic/1223@test.localhost")).to eq(true)
+      expect(check_format("<topic/1223@test.localhost>")).to eq(true)
+
+      expect(check_format("topic/1223@blah")).to eq(false)
+      expect(check_format("<CAPGrNgZ7QEFuPcsxJBRZLhBhAYPO_ruYpCANSdqiQEbc9Otpiw@mail.gmail.com>")).to eq(false)
+      expect(check_format("t/1223@test.localhost")).to eq(false)
+    end
+  end
+
+  describe "#message_id_rfc_format" do
+    it "returns message ID in RFC format" do
+      expect(Email::MessageIdService.message_id_rfc_format("test@test")).to eq("<test@test>")
+    end
+
+    it "returns input if already in RFC format" do
+      expect(Email::MessageIdService.message_id_rfc_format("<test@test>")).to eq("<test@test>")
+    end
+  end
+
+  describe "#message_id_clean" do
+    it "returns message ID if in RFC format" do
+      expect(Email::MessageIdService.message_id_clean("<test@test>")).to eq("test@test")
+    end
+
+    it "returns input if a clean message ID is not in RFC format" do
+      message_id = "<" + "@" * 50
+      expect(Email::MessageIdService.message_id_clean(message_id)).to eq(message_id)
+    end
+  end
+end