diff --git a/conservancy/usethesource/tests.py b/conservancy/usethesource/tests.py
new file mode 100644
index 0000000000000000000000000000000000000000..e13d83edbcd33405b1f85cc09f53b58912c49494
--- /dev/null
+++ b/conservancy/usethesource/tests.py
@@ -0,0 +1,56 @@
+import datetime
+import re
+
+from django.contrib.auth.models import User
+import pytest
+
+from . import models
+from .emails import make_comment_email
+from .models import Candidate, Comment
+
+
+def make_candidate(save=False, **kwargs):
+    defaults = {
+        'name': 'Test Candidate',
+        'slug': 'test',
+        'vendor': 'test vendor',
+        'device': 'test device',
+        'release_date': datetime.datetime(2000, 1, 1, tzinfo=datetime.timezone.utc)
+    }
+    merged = defaults | kwargs
+    obj = Candidate(**merged)
+    if save:
+        obj.save()
+    return obj
+
+
+def test_message_id():
+    assert re.match(r'<.+@.+>', models.gen_message_id())
+
+
+@pytest.mark.django_db
+def test_comment_knows_comment_its_replying_to():
+    user = User.objects.create()
+    candidate = make_candidate(name='Test Candidate', save=True)
+    first_comment = Comment.objects.create(user=user, candidate=candidate)
+    second_comment = Comment.objects.create(user=user, candidate=candidate)
+    assert second_comment._find_previous_comment() == first_comment
+    assert second_comment.in_reply_to() == first_comment.email_message_id
+
+
+@pytest.mark.django_db
+def test_comment_email():
+    user = User.objects.create(first_name='Test', last_name='User')
+    candidate = make_candidate(name='Test Candidate', save=True)
+    models.Comment.objects.create(candidate=candidate, user=user)
+    second_comment = models.Comment.objects.create(
+        candidate=candidate,
+        user=user,
+        message='Test message',
+    )
+    email = make_comment_email(second_comment)
+    assert 'Message-ID' in email.extra_headers
+    assert 'In-Reply-To' in email.extra_headers
+    assert email.subject == 'Re: Test Candidate'
+    assert 'Test message' in email.body
+    assert 'Test User' in email.body