diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0eb15ea..f53b148 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## NOT RELEASED
 
+### Changed
+
+- Add `Accept: application/json` header in request to fix incompatibility with 3rd party providers
+
 ## 1.8.0
 
 ### Added
diff --git a/src/Input/SendEmailRequest.php b/src/Input/SendEmailRequest.php
index a7b642d..d6d8f87 100644
--- a/src/Input/SendEmailRequest.php
+++ b/src/Input/SendEmailRequest.php
@@ -234,7 +234,10 @@ public function getReplyToAddresses(): array
     public function request(): Request
     {
         // Prepare headers
-        $headers = ['content-type' => 'application/json'];
+        $headers = [
+            'Content-Type' => 'application/json',
+            'Accept' => 'application/json',
+        ];
 
         // Prepare query
         $query = [];
diff --git a/tests/Unit/Input/SendEmailRequestTest.php b/tests/Unit/Input/SendEmailRequestTest.php
index 0c86c81..3f0fb40 100644
--- a/tests/Unit/Input/SendEmailRequestTest.php
+++ b/tests/Unit/Input/SendEmailRequestTest.php
@@ -45,7 +45,8 @@ public function testRequest(): void
         // see example-1.json from SDK
         $expected = '
             POST /v2/email/outbound-emails HTTP/1.0
-            Content-Type: application/json
+            Content-type: application/json
+            Accept: application/json
 
             {
                 "FromEmailAddress": "jeremy@derusse.com",