fix: preserve In-Reply-To/References via MAPI extended properties

CHANGELOG.md

@@ -5,6 +5,16 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- `In-Reply-To` and `References` headers are no longer dropped when sending.
+  The JSON `sendMail` payload cannot carry standard internet message headers,
+  so they are now mapped to their MAPI extended properties
+  (`PidTagInReplyToId` / `PidTagInternetReferences`), keeping replies threaded
+  in the recipient's mail client.
+
 ## [1.0.0] - 2025-12-15
 
 ### Added

lib/ms/graph/mailer/delivery_method.rb

@@ -4,6 +4,10 @@ module MsGraphMailer
   class DeliveryMethod
     include Dry::Monads[:result]
 
+    # PidTagInReplyToId (0x1042) / PidTagInternetReferences (0x1039)
+    IN_REPLY_TO_PROPERTY_ID = 'String 0x1042'
+    REFERENCES_PROPERTY_ID = 'String 0x1039'
+
     attr_reader :settings
 
     # INFO: settings is passed from config.action_mailer.<environment>
@@ -76,14 +80,42 @@ def build_message_payload(mail, sender_email)
         ccRecipients: build_recipients(mail.cc),
         bccRecipients: build_recipients(mail.bcc),
         replyTo: build_recipients(mail.reply_to),
-        attachments: build_attachments(mail)
+        attachments: build_attachments(mail),
+        singleValueExtendedProperties: build_threading_properties(mail)
       }.tap do |msg|
-        %i[toRecipients ccRecipients bccRecipients replyTo attachments].each do |field|
+        %i[toRecipients ccRecipients bccRecipients replyTo attachments singleValueExtendedProperties].each do |field|
           msg.delete(field) if msg[field].blank?
         end
       end
     end
 
+    # Graph's sendMail only accepts x-* custom headers via
+    # internetMessageHeaders, so the standard threading headers
+    # (In-Reply-To/References) would silently get lost. Mapping them to their
+    # MAPI extended properties makes Exchange emit them on the wire, keeping
+    # replies threaded in the recipient's client.
+    def build_threading_properties(mail)
+      properties = []
+
+      in_reply_to = format_message_ids(mail.in_reply_to)
+      references = format_message_ids(mail.references)
+
+      properties << { id: IN_REPLY_TO_PROPERTY_ID, value: in_reply_to } if in_reply_to
+      properties << { id: REFERENCES_PROPERTY_ID, value: references } if references
+      properties
+    end
+
+    # The mail gem parses message-id fields to bare ids (no angle brackets);
+    # the RFC 5322 header format Exchange writes needs them re-added.
+    def format_message_ids(ids)
+      formatted = Array(ids).compact.map(&:to_s).reject(&:empty?).map do |id|
+        id.start_with?('<') ? id : "<#{id}>"
+      end
+      return nil if formatted.empty?
+
+      formatted.join(' ')
+    end
+
     def build_body(mail)
       if mail.multipart?
         html_part = mail.html_part

spec/microsoft/graph/mailer/delivery_method_spec.rb

@@ -2,7 +2,89 @@
 
 require 'spec_helper'
 require 'ms/graph/mailer'
+require 'mail'
 
 RSpec.describe MsGraphMailer::DeliveryMethod do
-  # Add specs here
+  subject(:delivery_method) { described_class.new({}) }
+
+  let(:token_service) { instance_double(MsGraphMailer::TokenService) }
+  let(:send_mail_endpoint) { 'https://graph.microsoft.com/v1.0/users/sender%40example.com/sendMail' }
+
+  before do
+    MsGraphMailer.configure do |config|
+      config.tenant_id = 'tenant-id'
+      config.client_id = 'client-id'
+      config.client_secret = 'client-secret'
+      config.logger = Logger.new(File::NULL)
+    end
+
+    allow(MsGraphMailer::TokenService).to receive(:new).and_return(token_service)
+    allow(token_service).to receive(:call).and_return(Dry::Monads::Success('access-token'))
+
+    stub_request(:post, send_mail_endpoint).to_return(status: 202, body: '')
+  end
+
+  def build_mail(headers = {})
+    mail = Mail.new do
+      from 'sender@example.com'
+      to 'recipient@example.com'
+      subject 'Re: Hello'
+      body 'Hi there'
+    end
+    headers.each { |name, value| mail[name] = value }
+    mail
+  end
+
+  def sent_message_payload
+    payload = nil
+    expect(WebMock).to(have_requested(:post, send_mail_endpoint).with do |request|
+      payload = JSON.parse(request.body)['message']
+      true
+    end)
+    payload
+  end
+
+  describe '#deliver! threading headers' do
+    it 'transmits In-Reply-To and References as MAPI extended properties' do
+      mail = build_mail('In-Reply-To' => '<parent@example.com>',
+                        'References' => '<root@example.com> <parent@example.com>')
+
+      delivery_method.deliver!(mail)
+
+      properties = sent_message_payload['singleValueExtendedProperties']
+      expect(properties).to contain_exactly(
+        { 'id' => 'String 0x1042', 'value' => '<parent@example.com>' },
+        { 'id' => 'String 0x1039', 'value' => '<root@example.com> <parent@example.com>' }
+      )
+    end
+
+    it 'wraps bare message ids in angle brackets' do
+      mail = build_mail
+      mail.in_reply_to = 'parent@example.com'
+
+      delivery_method.deliver!(mail)
+
+      properties = sent_message_payload['singleValueExtendedProperties']
+      expect(properties).to contain_exactly(
+        { 'id' => 'String 0x1042', 'value' => '<parent@example.com>' }
+      )
+    end
+
+    it 'omits the extended properties when no threading headers are present' do
+      delivery_method.deliver!(build_mail)
+
+      expect(sent_message_payload).not_to have_key('singleValueExtendedProperties')
+    end
+  end
+
+  describe '#deliver! basic payload' do
+    it 'sends subject, body and recipients' do
+      delivery_method.deliver!(build_mail)
+
+      payload = sent_message_payload
+      expect(payload['subject']).to eq('Re: Hello')
+      expect(payload['body']).to include('content' => 'Hi there')
+      expect(payload['toRecipients']).to eq([{ 'emailAddress' => { 'address' => 'recipient@example.com' } }])
+    end
+  end
 end