diff --git a/test/escalated/services/email/inbound/parser_equivalence_test.exs b/test/escalated/services/email/inbound/parser_equivalence_test.exs
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+++ b/test/escalated/services/email/inbound/parser_equivalence_test.exs
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+defmodule Escalated.Services.Email.Inbound.ParserEquivalenceTest do
+  @moduledoc """
+  Parser-equivalence tests: the same logical email, expressed in each
+  provider's native webhook payload shape, should normalize to the
+  same `Escalated.Services.Email.Inbound.Message` metadata. Parser
+  equivalence at this layer guarantees a reply delivered via any
+  provider routes to the same ticket via the same threading chain.
+
+  Mirrors escalated-go#37 + escalated-dotnet#31 + escalated-spring#34.
+  Adding a fourth provider in the future can reuse the same
+  `logical_email/0` + `*_payload/1` builders and get contract
+  validation for free.
+  """
+
+  use ExUnit.Case, async: true
+
+  alias Escalated.Services.Email.Inbound.{
+    MailgunParser,
+    PostmarkParser,
+    SESParser
+  }
+
+  @sample %{
+    from_email: "alice@example.com",
+    from_name: "Alice",
+    to_email: "support@example.com",
+    subject: "Re: Help with invoice",
+    body_text: "Thanks for the quick response.",
+    message_id: "<external-reply-xyz@mail.alice.com>",
+    in_reply_to: "<ticket-42@support.example.com>",
+    references: "<ticket-42@support.example.com>"
+  }
+
+  defp postmark_payload(e) do
+    %{
+      "FromFull" => %{"Email" => e.from_email, "Name" => e.from_name},
+      "To" => e.to_email,
+      "Subject" => e.subject,
+      "TextBody" => e.body_text,
+      "Headers" => [
+        %{"Name" => "Message-ID", "Value" => e.message_id},
+        %{"Name" => "In-Reply-To", "Value" => e.in_reply_to},
+        %{"Name" => "References", "Value" => e.references}
+      ]
+    }
+  end
+
+  defp mailgun_payload(e) do
+    %{
+      "sender" => e.from_email,
+      "from" => "#{e.from_name} <#{e.from_email}>",
+      "recipient" => e.to_email,
+      "subject" => e.subject,
+      "body-plain" => e.body_text,
+      "Message-Id" => e.message_id,
+      "In-Reply-To" => e.in_reply_to,
+      "References" => e.references
+    }
+  end
+
+  defp ses_payload(e) do
+    # Include full raw MIME as base64 so body extraction is exercised
+    # — keeps the payload close to a real SES delivery.
+    mime =
+      "From: #{e.from_name} <#{e.from_email}>\r\n" <>
+        "To: #{e.to_email}\r\n" <>
+        "Subject: #{e.subject}\r\n" <>
+        "Message-ID: #{e.message_id}\r\n" <>
+        "In-Reply-To: #{e.in_reply_to}\r\n" <>
+        "References: #{e.references}\r\n" <>
+        "Content-Type: text/plain; charset=\"utf-8\"\r\n" <>
+        "\r\n" <>
+        e.body_text
+
+    ses_message = %{
+      "notificationType" => "Received",
+      "mail" => %{
+        "source" => e.from_email,
+        "destination" => [e.to_email],
+        "headers" => [
+          %{"name" => "From", "value" => "#{e.from_name} <#{e.from_email}>"},
+          %{"name" => "To", "value" => e.to_email},
+          %{"name" => "Subject", "value" => e.subject},
+          %{"name" => "Message-ID", "value" => e.message_id},
+          %{"name" => "In-Reply-To", "value" => e.in_reply_to},
+          %{"name" => "References", "value" => e.references}
+        ],
+        "commonHeaders" => %{
+          "from" => ["#{e.from_name} <#{e.from_email}>"],
+          "to" => [e.to_email],
+          "subject" => e.subject
+        }
+      },
+      "content" => Base.encode64(mime)
+    }
+
+    %{
+      "Type" => "Notification",
+      "Message" => Jason.encode!(ses_message)
+    }
+  end
+
+  describe "normalizes to same message" do
+    test "threading metadata matches across Postmark / Mailgun / SES" do
+      {:ok, postmark} = PostmarkParser.parse(postmark_payload(@sample))
+      {:ok, mailgun} = MailgunParser.parse(mailgun_payload(@sample))
+      {:ok, ses} = SESParser.parse(ses_payload(@sample))
+
+      for {name, msg} <- [{"postmark", postmark}, {"mailgun", mailgun}, {"ses", ses}] do
+        assert msg.from_email == @sample.from_email, "#{name}: from_email mismatch"
+        assert msg.to_email == @sample.to_email, "#{name}: to_email mismatch"
+        assert msg.subject == @sample.subject, "#{name}: subject mismatch"
+        assert msg.in_reply_to == @sample.in_reply_to, "#{name}: in_reply_to mismatch"
+        assert msg.references == @sample.references, "#{name}: references mismatch"
+      end
+    end
+  end
+
+  describe "body extraction matches" do
+    test "body_text matches across all three parsers" do
+      {:ok, postmark} = PostmarkParser.parse(postmark_payload(@sample))
+      {:ok, mailgun} = MailgunParser.parse(mailgun_payload(@sample))
+      {:ok, ses} = SESParser.parse(ses_payload(@sample))
+
+      assert postmark.body_text == @sample.body_text
+      assert mailgun.body_text == @sample.body_text
+      assert ses.body_text == @sample.body_text
+    end
+  end
+end