diff --git a/h/emails/flag_notification.py b/h/emails/flag_notification.py
index c672d972deb..e1652def4ce 100644
--- a/h/emails/flag_notification.py
+++ b/h/emails/flag_notification.py
@@ -1,6 +1,7 @@
 from pyramid.renderers import render
 
 from h.i18n import TranslationString as _
+from h.services.email import EmailTag
 
 
 def generate(request, email, incontext_link):
@@ -27,4 +28,4 @@ def generate(request, email, incontext_link):
         "h:templates/emails/flag_notification.html.jinja2", context, request=request
     )
 
-    return [email], subject, text, html
+    return [email], subject, text, EmailTag.FLAG_NOTIFICATION, html
diff --git a/h/emails/reply_notification.py b/h/emails/reply_notification.py
index 078ce971962..71388527d30 100644
--- a/h/emails/reply_notification.py
+++ b/h/emails/reply_notification.py
@@ -5,6 +5,7 @@
 from h.models import Subscriptions
 from h.notification.reply import Notification
 from h.services import SubscriptionService
+from h.services.email import EmailTag
 
 
 def generate(request: Request, notification: Notification):
@@ -49,7 +50,13 @@ def generate(request: Request, notification: Notification):
         "h:templates/emails/reply_notification.html.jinja2", context, request=request
     )
 
-    return [notification.parent_user.email], subject, text, html
+    return (
+        [notification.parent_user.email],
+        subject,
+        text,
+        EmailTag.REPLY_NOTIFICATION,
+        html
+    )
 
 
 def _get_user_url(user, request):
diff --git a/h/emails/reset_password.py b/h/emails/reset_password.py
index fd81816e191..d0fe8265fd7 100644
--- a/h/emails/reset_password.py
+++ b/h/emails/reset_password.py
@@ -1,6 +1,7 @@
 from pyramid.renderers import render
 
 from h.i18n import TranslationString as _
+from h.services.email import EmailTag
 
 
 def generate(request, user):
@@ -31,4 +32,4 @@ def generate(request, user):
         "h:templates/emails/reset_password.html.jinja2", context, request=request
     )
 
-    return [user.email], subject, text, html
+    return [user.email], subject, text, EmailTag.RESET_PASSWORD, html
diff --git a/h/emails/signup.py b/h/emails/signup.py
index 1df59e50c2b..60928fb14d0 100644
--- a/h/emails/signup.py
+++ b/h/emails/signup.py
@@ -1,6 +1,7 @@
 from pyramid.renderers import render
 
 from h.i18n import TranslationString as _
+from h.services.email import EmailTag
 
 
 def generate(request, user_id, email, activation_code):
@@ -27,4 +28,4 @@ def generate(request, user_id, email, activation_code):
     text = render("h:templates/emails/signup.txt.jinja2", context, request=request)
     html = render("h:templates/emails/signup.html.jinja2", context, request=request)
 
-    return [email], subject, text, html
+    return [email], subject, text, EmailTag.ACTIVATION, html
diff --git a/h/emails/test.py b/h/emails/test.py
index fbe430af030..a362f2626bf 100644
--- a/h/emails/test.py
+++ b/h/emails/test.py
@@ -4,6 +4,7 @@
 from pyramid.renderers import render
 
 from h import __version__
+from h.services.email import EmailTag
 
 
 def generate(request, recipient):
@@ -28,4 +29,4 @@ def generate(request, recipient):
     text = render("h:templates/emails/test.txt.jinja2", context, request=request)
     html = render("h:templates/emails/test.html.jinja2", context, request=request)
 
-    return [recipient], "Test mail", text, html
+    return [recipient], "Test mail", text, EmailTag.TEST, html
diff --git a/h/services/email.py b/h/services/email.py
index 34d7ee09f5f..11c29b18fe4 100644
--- a/h/services/email.py
+++ b/h/services/email.py
@@ -1,6 +1,7 @@
 # noqa: A005
 
 import smtplib
+from enum import StrEnum
 
 import pyramid_mailer
 import pyramid_mailer.message
@@ -12,6 +13,14 @@
 logger = get_task_logger(__name__)
 
 
+class EmailTag(StrEnum):
+    ACTIVATION = "activation"
+    FLAG_NOTIFICATION = "flag_notification"
+    REPLY_NOTIFICATION = "reply_notification"
+    RESET_PASSWORD = "reset_password"  # noqa: S105
+    TEST = "test"
+
+
 class EmailService:
     """A service for sending emails."""
 
@@ -20,10 +29,20 @@ def __init__(self, request: Request, mailer: IMailer) -> None:
         self._mailer = mailer
 
     def send(
-        self, recipients: list[str], subject: str, body: str, html: str | None = None
-    ):
+        self,
+        recipients: list[str],
+        subject: str,
+        body: str,
+        tag: EmailTag,
+        html: str | None = None,
+    ) -> None:
+        extra_headers = {"X-MC-Tags": tag}
         email = pyramid_mailer.message.Message(
-            subject=subject, recipients=recipients, body=body, html=html
+            subject=subject,
+            recipients=recipients,
+            body=body,
+            html=html,
+            extra_headers=extra_headers,
         )
         if self._request.debug:  # pragma: no cover
             logger.info("emailing in debug mode: check the `mail/` directory")
diff --git a/h/tasks/mailer.py b/h/tasks/mailer.py
index bf4ee51af2e..08e3c03d2e5 100644
--- a/h/tasks/mailer.py
+++ b/h/tasks/mailer.py
@@ -6,14 +6,21 @@
 
 import smtplib
 
-from h.services.email import EmailService
+from h.services.email import EmailService, EmailTag
 from h.tasks.celery import celery
 
 __all__ = ("send",)
 
 
 @celery.task(bind=True, max_retries=3, acks_late=True)
-def send(self, recipients, subject, body, html=None):
+def send(  # noqa: PLR0913
+    self,
+    tag: EmailTag,
+    recipients: list[str],
+    subject: str,
+    body: str,
+    html: str | None = None,
+) -> None:
     """
     Send an email.
 
@@ -28,7 +35,9 @@ def send(self, recipients, subject, body, html=None):
     """
     service = celery.request.find_service(EmailService)
     try:
-        service.send(recipients=recipients, subject=subject, body=body, html=html)
+        service.send(
+            tag=tag, recipients=recipients, subject=subject, body=body, html=html
+        )
     except smtplib.socket.error as exc:
         # Exponential backoff in case the SMTP service is having problems.
         countdown = self.default_retry_delay * 2**self.request.retries
diff --git a/tests/unit/h/emails/flag_notification_test.py b/tests/unit/h/emails/flag_notification_test.py
index 86a3e7e2038..ce467fbe90c 100644
--- a/tests/unit/h/emails/flag_notification_test.py
+++ b/tests/unit/h/emails/flag_notification_test.py
@@ -1,6 +1,7 @@
 import pytest
 
 from h.emails.flag_notification import generate
+from h.services.email import EmailTag
 
 
 class TestGenerate:
@@ -23,7 +24,7 @@ def test_appropriate_return_values(
         html_renderer.string_response = "HTML output"
         text_renderer.string_response = "Text output"
 
-        recipients, subject, text, html = generate(
+        recipients, subject, text, tag, html = generate(
             pyramid_request,
             email="foo@example.com",
             incontext_link="http://hyp.is/a/ann1",
@@ -32,6 +33,7 @@ def test_appropriate_return_values(
         assert recipients == ["foo@example.com"]
         assert subject == "An annotation has been flagged"
         assert html == "HTML output"
+        assert tag == EmailTag.FLAG_NOTIFICATION
         assert text == "Text output"
 
     @pytest.fixture
diff --git a/tests/unit/h/emails/reply_notification_test.py b/tests/unit/h/emails/reply_notification_test.py
index af634e6dea2..9ca3591db1f 100644
--- a/tests/unit/h/emails/reply_notification_test.py
+++ b/tests/unit/h/emails/reply_notification_test.py
@@ -99,7 +99,7 @@ def test_supports_non_ascii_display_names(
         parent_user.display_name = "Parent 👩"
         reply_user.display_name = "Child 👧"
 
-        (_, subject, _, _) = generate(pyramid_request, notification)
+        (_, subject, _, _, _) = generate(pyramid_request, notification)
 
         assert subject == "Child 👧 has replied to your annotation"
 
@@ -130,7 +130,7 @@ def test_returns_text_and_body_results_from_renderers(
         html_renderer.string_response = "HTML output"
         text_renderer.string_response = "Text output"
 
-        _, _, text, html = generate(pyramid_request, notification)
+        _, _, text, _, html = generate(pyramid_request, notification)
 
         assert html == "HTML output"
         assert text == "Text output"
@@ -138,7 +138,7 @@ def test_returns_text_and_body_results_from_renderers(
     def test_returns_subject_with_reply_display_name(
         self, notification, pyramid_request
     ):
-        _, subject, _, _ = generate(pyramid_request, notification)
+        _, subject, _, _, _ = generate(pyramid_request, notification)
 
         assert subject == "Ron Burgundy has replied to your annotation"
 
@@ -146,12 +146,12 @@ def test_returns_subject_with_reply_username(
         self, notification, pyramid_request, reply_user
     ):
         reply_user.display_name = None
-        _, subject, _, _ = generate(pyramid_request, notification)
+        _, subject, _, _, _ = generate(pyramid_request, notification)
 
         assert subject == "ron has replied to your annotation"
 
     def test_returns_parent_email_as_recipients(self, notification, pyramid_request):
-        recipients, _, _, _ = generate(pyramid_request, notification)
+        recipients, _, _, _, _ = generate(pyramid_request, notification)
 
         assert recipients == ["pat@ric.ia"]
 
diff --git a/tests/unit/h/emails/reset_password_test.py b/tests/unit/h/emails/reset_password_test.py
index d70d8b5ff05..fc2adcfa1b7 100644
--- a/tests/unit/h/emails/reset_password_test.py
+++ b/tests/unit/h/emails/reset_password_test.py
@@ -3,6 +3,7 @@
 import pytest
 
 from h.emails.reset_password import generate
+from h.services.email import EmailTag
 
 
 @pytest.mark.usefixtures("routes")
@@ -38,11 +39,12 @@ def test_appropriate_return_values(
         html_renderer.string_response = "HTML output"
         text_renderer.string_response = "Text output"
 
-        recipients, subject, text, html = generate(pyramid_request, user)
+        recipients, subject, text, tag, html = generate(pyramid_request, user)
 
         assert recipients == [user.email]
         assert subject == "Reset your password"
         assert html == "HTML output"
+        assert tag == EmailTag.RESET_PASSWORD
         assert text == "Text output"
 
     def test_jinja_templates_render(
diff --git a/tests/unit/h/emails/signup_test.py b/tests/unit/h/emails/signup_test.py
index 2e690c18494..cbafc12227e 100644
--- a/tests/unit/h/emails/signup_test.py
+++ b/tests/unit/h/emails/signup_test.py
@@ -1,6 +1,7 @@
 import pytest
 
 from h.emails.signup import generate
+from h.services.email import EmailTag
 
 
 @pytest.mark.usefixtures("routes")
@@ -27,7 +28,7 @@ def test_appropriate_return_values(
         html_renderer.string_response = "HTML output"
         text_renderer.string_response = "Text output"
 
-        recipients, subject, text, html = generate(
+        recipients, subject, text, tag, html = generate(
             pyramid_request,
             user_id=1234,
             email="foo@example.com",
@@ -37,6 +38,7 @@ def test_appropriate_return_values(
         assert recipients == ["foo@example.com"]
         assert subject == "Please activate your account"
         assert html == "HTML output"
+        assert tag == EmailTag.ACTIVATION
         assert text == "Text output"
 
     def test_jinja_templates_render(self, pyramid_config, pyramid_request):
diff --git a/tests/unit/h/emails/test_test.py b/tests/unit/h/emails/test_test.py
index 1ae9495ab01..18c968c4827 100644
--- a/tests/unit/h/emails/test_test.py
+++ b/tests/unit/h/emails/test_test.py
@@ -3,6 +3,7 @@
 
 from h import __version__
 from h.emails.test import generate
+from h.services.email import EmailTag
 
 
 class TestGenerate:
@@ -26,13 +27,14 @@ def test_appropriate_return_values(
         html_renderer.string_response = "HTML output"
         text_renderer.string_response = "Text output"
 
-        recipients, subject, text, html = generate(
+        recipients, subject, text, tag, html = generate(
             pyramid_request, "meerkat@example.com"
         )
 
         assert recipients == ["meerkat@example.com"]
         assert subject == "Test mail"
         assert html == "HTML output"
+        assert tag == EmailTag.TEST
         assert text == "Text output"
 
     def test_jinja_templates_render(self, pyramid_config, pyramid_request):
diff --git a/tests/unit/h/services/email_test.py b/tests/unit/h/services/email_test.py
index 83c2c1c08d1..92905168306 100644
--- a/tests/unit/h/services/email_test.py
+++ b/tests/unit/h/services/email_test.py
@@ -3,7 +3,7 @@
 
 import pytest
 
-from h.services.email import EmailService, factory
+from h.services.email import EmailService, EmailTag, factory
 
 
 class TestEmailService:
@@ -12,6 +12,7 @@ def test_send_creates_email_message(self, email_service, pyramid_mailer):
             recipients=["foo@example.com"],
             subject="My email subject",
             body="Some text body",
+            tag=EmailTag.TEST,
         )
 
         pyramid_mailer.message.Message.assert_called_once_with(
@@ -19,6 +20,7 @@ def test_send_creates_email_message(self, email_service, pyramid_mailer):
             subject="My email subject",
             body="Some text body",
             html=None,
+            extra_headers={"X-MC-Tags": EmailTag.TEST},
         )
 
     def test_send_creates_email_message_with_html_body(
@@ -28,6 +30,7 @@ def test_send_creates_email_message_with_html_body(
             recipients=["foo@example.com"],
             subject="My email subject",
             body="Some text body",
+            tag=EmailTag.TEST,
             html="<p>An HTML body</p>",
         )
 
@@ -36,6 +39,7 @@ def test_send_creates_email_message_with_html_body(
             subject="My email subject",
             body="Some text body",
             html="<p>An HTML body</p>",
+            extra_headers={"X-MC-Tags": EmailTag.TEST},
         )
 
     def test_send_dispatches_email_using_request_mailer(
@@ -48,6 +52,7 @@ def test_send_dispatches_email_using_request_mailer(
             recipients=["foo@example.com"],
             subject="My email subject",
             body="Some text body",
+            tag=EmailTag.TEST,
         )
 
         request_mailer.send_immediately.assert_called_once_with(message)
@@ -61,6 +66,7 @@ def test_raises_smtplib_exception(self, email_service, pyramid_mailer):
                 recipients=["foo@example.com"],
                 subject="My email subject",
                 body="Some text body",
+                tag=EmailTag.TEST,
             )
 
     @pytest.fixture
diff --git a/tests/unit/h/tasks/mailer_test.py b/tests/unit/h/tasks/mailer_test.py
index e3e8330796d..fae4abddbed 100644
--- a/tests/unit/h/tasks/mailer_test.py
+++ b/tests/unit/h/tasks/mailer_test.py
@@ -3,6 +3,7 @@
 
 import pytest
 
+from h.services.email import EmailTag
 from h.tasks import mailer
 
 
@@ -14,6 +15,7 @@ def test_send_retries_if_mailing_fails(email_service):
         recipients=["foo@example.com"],
         subject="My email subject",
         body="Some text body",
+        tag=EmailTag.TEST,
     )
 
     assert mailer.send.retry.called