diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index e59e2a5..f771ee6 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -33,8 +33,8 @@ vNext
 Fixes
 ~~~~~
 
-* **Postmark:** Fix Postmark API error when sending with a template that doesn't
-  require any merge data. (Thanks to `@Tobeyforce`_ for reporting it.)
+* **Postmark:** Fix two different errors when sending with a template but no merge
+  data. (Thanks to `@kareemcoding`_ and `@Tobeyforce`_ for reporting them.)
 
 
 v8.2
@@ -1226,6 +1226,7 @@ Features
 .. _@janneThoft: https://github.com/janneThoft
 .. _@jc-ee: https://github.com/jc-ee
 .. _@joshkersey: https://github.com/joshkersey
+.. _@kareemcoding: https://github.com/kareemcoding
 .. _@kika115: https://github.com/kika115
 .. _@Lekensteyn: https://github.com/Lekensteyn
 .. _@lewistaylor: https://github.com/lewistaylor
diff --git a/anymail/backends/postmark.py b/anymail/backends/postmark.py
index 9ca82a4..05535e2 100644
--- a/anymail/backends/postmark.py
+++ b/anymail/backends/postmark.py
@@ -160,7 +160,7 @@ class PostmarkPayload(RequestsPayload):
         super().__init__(message, defaults, backend, headers=headers, *args, **kwargs)
 
     def get_api_endpoint(self):
-        batch_send = self.is_batch() and len(self.to_emails) > 1
+        batch_send = self.is_batch()
         if 'TemplateAlias' in self.data or 'TemplateId' in self.data or 'TemplateModel' in self.data:
             if batch_send:
                 return "email/batchWithTemplates"
@@ -187,8 +187,8 @@ class PostmarkPayload(RequestsPayload):
         elif api_endpoint == "email/batch":
             data = [self.data_for_recipient(to) for to in self.to_emails]
         elif api_endpoint == "email/withTemplate/":
-            assert len(self.to_emails) == 1
-            data = self.data_for_recipient(self.to_emails[0])
+            assert self.merge_data is None and self.merge_metadata is None  # else it's a batch send
+            data = self.data
         else:
             raise AssertionError("PostmarkPayload.serialize_data missing"
                                  " case for api_endpoint %r" % api_endpoint)
diff --git a/tests/test_postmark_backend.py b/tests/test_postmark_backend.py
index 5f10921..9c0fc42 100644
--- a/tests/test_postmark_backend.py
+++ b/tests/test_postmark_backend.py
@@ -398,6 +398,19 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         # Postmark requires TemplateModel (can be empty) with TemplateId/TemplateAlias
         self.assertEqual(data['TemplateModel'], {})
 
+    def test_template_multiple_recipients(self):
+        # This is a non-batch (no merge_data) template send
+        message = AnymailMessage(
+            from_email='from@example.com',
+            to=['to@example.com', "Also to <to2@example.com>"],
+            template_id=1234567,
+        )
+        message.send()
+        self.assert_esp_called('/email/withTemplate/')
+        data = self.get_api_call_json()
+        self.assertEqual(data['To'], 'to@example.com, Also to <to2@example.com>')
+        self.assertEqual(data['TemplateId'], 1234567)
+
     _mock_batch_response = json.dumps([{
             "ErrorCode": 0,
             "Message": "OK",
@@ -464,14 +477,17 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         )
         message.send()
 
-        self.assert_esp_called('/email/withTemplate/')
+        # because merge_data is set, it's treated as a batch send
+        self.assert_esp_called('/email/batchWithTemplates')
         data = self.get_api_call_json()
 
         self.assertEqual(data, {
-            "From": "from@example.com",
-            "To": "alice@example.com",
-            "TemplateId": 1234567,
-            "TemplateModel": {"name": "Alice", "group": "Developers", "site": "ExampleCo"},
+            'Messages': [{
+                "From": "from@example.com",
+                "To": "alice@example.com",
+                "TemplateId": 1234567,
+                "TemplateModel": {"name": "Alice", "group": "Developers", "site": "ExampleCo"},
+            }]
         })
 
         recipients = message.anymail_status.recipients