diff --git a/src/Postmark/PostmarkClient.php b/src/Postmark/PostmarkClient.php
index 53652d51..1e085244 100644
--- a/src/Postmark/PostmarkClient.php
+++ b/src/Postmark/PostmarkClient.php
@@ -129,6 +129,8 @@ public function sendEmail(
      */
     public function sendEmailModel(PostmarkMessage $postmarkMessage): PostmarkResponse
     {
+        $headers = $this->fixHeaders($postmarkMessage->getHeaders());
+        $postmarkMessage->setHeaders($headers);
         return new PostmarkResponse((array) $this->processRestRequest('POST', '/email', (array) $postmarkMessage));
     }
 
@@ -277,6 +279,8 @@ public function sendEmailWithTemplate(
 
     public function sendEmailWithTemplateModel(TemplatedPostmarkMessage $postmarkMessageTemplate): PostmarkResponse
     {
+        $headers = $this->fixHeaders($postmarkMessageTemplate->getHeaders());
+        $postmarkMessageTemplate->setHeaders($headers);
         return new PostmarkResponse((array) $this->processRestRequest('POST', '/email/withTemplate', (array) $postmarkMessageTemplate));
     }
 
diff --git a/tests/PostmarkClientEmailTest.php b/tests/PostmarkClientEmailTest.php
index 9d06cb63..ad676d3b 100644
--- a/tests/PostmarkClientEmailTest.php
+++ b/tests/PostmarkClientEmailTest.php
@@ -106,9 +106,10 @@ public function testClientSendModel()
         $emailModel->setFrom($tk->WRITE_TEST_SENDER_EMAIL_ADDRESS);
         $emailModel->setTo($tk->WRITE_TEST_EMAIL_RECIPIENT_ADDRESS);
         $emailModel->setSubject("Hello from the PHP Postmark Client Tests! ({$currentTime})");
-        $emailModel->setHtmlBody('<b>Hi there!</b>');
-        $emailModel->setTextBody('This is a text body for a test email.');
+        $emailModel->setHtmlBody('<b>Hi there! sent via a model.</b>');
+        $emailModel->setTextBody('This is a text body for a test email sent via a model.');
         $emailModel->setMessageStream('outbound');
+        $emailModel->setHeaders(['X-Test-Header' => 'Header.', 'X-Test-Header-2' => 'Test Header 2']);
 
         $response = $client->sendEmailModel($emailModel);
 
diff --git a/tests/PostmarkClientTemplatesTest.php b/tests/PostmarkClientTemplatesTest.php
index 68fed1f1..0510542c 100644
--- a/tests/PostmarkClientTemplatesTest.php
+++ b/tests/PostmarkClientTemplatesTest.php
@@ -176,13 +176,14 @@ public function testClientCanSendMailWithTemplateModel()
     {
         $tk = parent::$testKeys;
         $client = new PostmarkClient($tk->WRITE_TEST_SERVER_TOKEN, $tk->TEST_TIMEOUT);
-        $result = $client->createTemplate('test-php-template-' . date('c'), '{{subject}}', 'Hello <b>{{name}}</b>!', 'Hello {{name}}!');
+        $result = $client->createTemplate('test-php-template-' . date('c'), '{{subject}}', 'Hello <b>{{name}}</b> from Template Model!', 'Hello {{name}} from Template Model!');
 
         $templatedModel = new TemplatedPostmarkMessage();
         $templatedModel->setFrom($tk->WRITE_TEST_SENDER_EMAIL_ADDRESS);
         $templatedModel->setTo($tk->WRITE_TEST_EMAIL_RECIPIENT_ADDRESS);
         $templatedModel->setTemplateId($result->getTemplateId());
         $templatedModel->setTemplateModel(['subjectValue' => 'Hello!']);
+        $templatedModel->setHeaders(['X-Test-Header' => 'Header.', 'X-Test-Header-2' => 'Test Header 2']);
 
         $emailResult = $client->sendEmailWithTemplateModel($templatedModel);