diff --git a/includes/Services/InboundEmailService.php b/includes/Services/InboundEmailService.php
index d66fe11..4efd0a4 100644
--- a/includes/Services/InboundEmailService.php
+++ b/includes/Services/InboundEmailService.php
@@ -150,30 +150,62 @@ public function process(array $message, string $adapter): ?object
      */
     public function find_ticket_by_email(array $message): ?object
     {
-        global $wpdb;
+        $header_ids = $this->candidate_header_message_ids($message);
+
+        // 1 + 2. Parse our own Message-IDs out of In-Reply-To /
+        // References via Message_Id_Util. Cold-start path — no DB
+        // lookup needed when the incoming thread references a
+        // Message-ID we issued.
+        foreach ($header_ids as $raw) {
+            $ticket_id = \Escalated\Mail\Message_Id_Util::parse_ticket_id_from_message_id($raw);
+            if ($ticket_id === null) {
+                continue;
+            }
+            $ticket = Ticket::find($ticket_id);
+            if ($ticket) {
+                return $ticket;
+            }
+        }
 
-        $subject = $message['subject'] ?? '';
+        // 3. Signed Reply-To on the recipient address
+        // (reply+{id}.{hmac8}@...) verified with Message_Id_Util.
+        // Survives clients that strip our threading headers; forged
+        // signatures are rejected.
+        $secret = \Escalated\Mail\Email_Threading::get_inbound_secret();
+        if ($secret !== '' && ! empty($message['toEmail'])) {
+            $verified = \Escalated\Mail\Message_Id_Util::verify_reply_to(
+                (string) $message['toEmail'],
+                $secret
+            );
+            if ($verified !== null) {
+                $ticket = Ticket::find($verified);
+                if ($ticket) {
+                    return $ticket;
+                }
+            }
+        }
 
-        // Check subject for ticket reference pattern like [ESC-00001].
+        // 4. Subject line reference tag (legacy).
+        $subject = $message['subject'] ?? '';
         if (preg_match('/\[([A-Z]+-\d{5,})\]/', $subject, $matches)) {
-            $reference = $matches[1];
-            $ticket = Ticket::find_by_reference($reference);
+            $ticket = Ticket::find_by_reference($matches[1]);
             if ($ticket) {
                 return $ticket;
             }
         }
 
-        // Check In-Reply-To header.
+        // 5. InboundEmail lookup (weakest fallback; relies on our own
+        // reply history). Ensures legacy deployments keep working
+        // during the Message-ID format transition.
+        global $wpdb;
         $inbound_table = Escalated::table('inbound_emails');
-
-        if (! empty($message['inReplyTo'])) {
+        foreach ($header_ids as $raw) {
             $related = $wpdb->get_row(
                 $wpdb->prepare(
                     "SELECT ticket_id FROM {$inbound_table} WHERE message_id = %s AND ticket_id IS NOT NULL LIMIT 1",
-                    $message['inReplyTo']
+                    $raw
                 )
             );
-
             if ($related && ! empty($related->ticket_id)) {
                 $ticket = Ticket::find((int) $related->ticket_id);
                 if ($ticket) {
@@ -182,35 +214,33 @@ public function find_ticket_by_email(array $message): ?object
             }
         }
 
-        // Check References header (may contain multiple message IDs).
+        return null;
+    }
+
+    /**
+     * Return every candidate Message-ID from the inbound headers.
+     *
+     * @return array<string>
+     */
+    protected function candidate_header_message_ids(array $message): array
+    {
+        $ids = [];
+        if (! empty($message['inReplyTo'])) {
+            $ids[] = (string) $message['inReplyTo'];
+        }
         if (! empty($message['references'])) {
-            $references = is_array($message['references'])
+            $refs = is_array($message['references'])
                 ? $message['references']
-                : preg_split('/\s+/', $message['references']);
-
-            foreach ($references as $ref_message_id) {
-                $ref_message_id = trim($ref_message_id);
-                if (empty($ref_message_id)) {
-                    continue;
-                }
-
-                $related = $wpdb->get_row(
-                    $wpdb->prepare(
-                        "SELECT ticket_id FROM {$inbound_table} WHERE message_id = %s AND ticket_id IS NOT NULL LIMIT 1",
-                        $ref_message_id
-                    )
-                );
-
-                if ($related && ! empty($related->ticket_id)) {
-                    $ticket = Ticket::find((int) $related->ticket_id);
-                    if ($ticket) {
-                        return $ticket;
-                    }
+                : preg_split('/\s+/', (string) $message['references']);
+            foreach ((array) $refs as $ref) {
+                $ref = trim((string) $ref);
+                if ($ref !== '') {
+                    $ids[] = $ref;
                 }
             }
         }
 
-        return null;
+        return $ids;
     }
 
     /**
diff --git a/tests/Test_Inbound_Email_Service.php b/tests/Test_Inbound_Email_Service.php
new file mode 100644
index 0000000..45dd190
--- /dev/null
+++ b/tests/Test_Inbound_Email_Service.php
@@ -0,0 +1,147 @@
+<?php
+
+/**
+ * Tests for InboundEmailService::find_ticket_by_email resolution order.
+ *
+ * Verifies that the Message_Id_Util wire-up resolves canonical
+ * Message-IDs out of In-Reply-To / References and verifies signed
+ * Reply-To addresses on the recipient.
+ */
+
+use Escalated\Mail\Message_Id_Util;
+use Escalated\Models\Ticket;
+use Escalated\Services\InboundEmailService;
+use Escalated\Services\TicketService;
+
+class Test_Inbound_Email_Service extends WP_UnitTestCase
+{
+    private InboundEmailService $service;
+
+    private TicketService $ticket_service;
+
+    private int $user_id;
+
+    public function set_up(): void
+    {
+        parent::set_up();
+
+        \Escalated\Activator::activate();
+
+        $this->service = new InboundEmailService;
+        $this->ticket_service = new TicketService;
+        $this->user_id = $this->factory->user->create(['role' => 'subscriber']);
+    }
+
+    public function tear_down(): void
+    {
+        delete_option('escalated_email_inbound_secret');
+        delete_option('escalated_email_domain');
+        parent::tear_down();
+    }
+
+    private function make_ticket(array $overrides = []): object
+    {
+        $defaults = [
+            'subject' => 'Test',
+            'description' => 'Body',
+            'priority' => 'medium',
+            'channel' => 'web',
+        ];
+
+        return $this->ticket_service->create($this->user_id, array_merge($defaults, $overrides));
+    }
+
+    private function make_message(array $overrides = []): array
+    {
+        return array_merge([
+            'fromEmail' => 'customer@example.com',
+            'fromName' => 'Customer',
+            'toEmail' => 'support@example.com',
+            'subject' => 'hello',
+            'bodyText' => 'body',
+        ], $overrides);
+    }
+
+    public function test_finds_ticket_by_canonical_in_reply_to(): void
+    {
+        $ticket = $this->make_ticket();
+        $message = $this->make_message([
+            'inReplyTo' => "<ticket-{$ticket->id}@support.example.com>",
+        ]);
+
+        $found = $this->service->find_ticket_by_email($message);
+        $this->assertNotNull($found);
+        $this->assertEquals($ticket->id, (int) $found->id);
+    }
+
+    public function test_finds_ticket_by_canonical_references_header(): void
+    {
+        $ticket = $this->make_ticket();
+        $message = $this->make_message([
+            'references' => "<unrelated@mail.com> <ticket-{$ticket->id}@support.example.com>",
+        ]);
+
+        $found = $this->service->find_ticket_by_email($message);
+        $this->assertNotNull($found);
+        $this->assertEquals($ticket->id, (int) $found->id);
+    }
+
+    public function test_finds_ticket_by_signed_reply_to(): void
+    {
+        update_option('escalated_email_domain', 'support.example.com');
+        update_option('escalated_email_inbound_secret', 'test-secret');
+        $ticket = $this->make_ticket();
+
+        $to = Message_Id_Util::build_reply_to(
+            (int) $ticket->id,
+            'test-secret',
+            'support.example.com'
+        );
+        $message = $this->make_message(['toEmail' => $to]);
+
+        $found = $this->service->find_ticket_by_email($message);
+        $this->assertNotNull($found);
+        $this->assertEquals($ticket->id, (int) $found->id);
+    }
+
+    public function test_rejects_forged_reply_to_signature(): void
+    {
+        update_option('escalated_email_domain', 'support.example.com');
+        update_option('escalated_email_inbound_secret', 'real-secret');
+        $ticket = $this->make_ticket();
+
+        $forged = Message_Id_Util::build_reply_to(
+            (int) $ticket->id,
+            'wrong-secret',
+            'support.example.com'
+        );
+        $message = $this->make_message(['toEmail' => $forged]);
+
+        $found = $this->service->find_ticket_by_email($message);
+        $this->assertNull($found);
+    }
+
+    public function test_ignores_signed_reply_to_when_secret_blank(): void
+    {
+        delete_option('escalated_email_inbound_secret');
+        $ticket = $this->make_ticket();
+
+        $to = Message_Id_Util::build_reply_to(
+            (int) $ticket->id,
+            'test-secret',
+            'support.example.com'
+        );
+        $message = $this->make_message(['toEmail' => $to]);
+
+        $found = $this->service->find_ticket_by_email($message);
+        $this->assertNull($found);
+    }
+
+    public function test_returns_null_when_nothing_matches(): void
+    {
+        $message = $this->make_message(['subject' => 'Completely unrelated']);
+
+        $found = $this->service->find_ticket_by_email($message);
+        $this->assertNull($found);
+    }
+}