diff --git a/README.md b/README.md
index d5bd5ed..a8832d6 100644
--- a/README.md
+++ b/README.md
@@ -60,14 +60,17 @@ Here's how to send a message using the SDK:
 $mg = Mailgun::create('key-example');
 
 # Now, compose and send your message.
+# $mg->messages()->send($domain, $params);
 $mg->messages()->send('example.com', [
-  'from'    => 'bob@example.com', 
-  'to'      => 'sally@example.com', 
-  'subject' => 'The PHP SDK is awesome!', 
+  'from'    => 'bob@example.com',
+  'to'      => 'sally@example.com',
+  'subject' => 'The PHP SDK is awesome!',
   'text'    => 'It is so simple to send a message.'
 ]);
 ```
 
+Attention: `$domain` must match to the domain you have configured on [app.mailgun.com](https://app.mailgun.com/app/domains).
+
 ### All usage examples
 
 You find more detailed documentation at in [/doc](doc/index.md) and on 
diff --git a/src/Mailgun/Constants/ExceptionMessages.php b/src/Mailgun/Constants/ExceptionMessages.php
index ab370aa..75f2aa4 100644
--- a/src/Mailgun/Constants/ExceptionMessages.php
+++ b/src/Mailgun/Constants/ExceptionMessages.php
@@ -18,7 +18,7 @@ class ExceptionMessages
     const EXCEPTION_GENERIC_HTTP_ERROR = 'An HTTP Error has occurred! Check your network connection and try again.';
     const EXCEPTION_MISSING_REQUIRED_PARAMETERS = 'The parameters passed to the API were invalid. Check your inputs!';
     const EXCEPTION_MISSING_REQUIRED_MIME_PARAMETERS = 'The parameters passed to the API were invalid. Check your inputs!';
-    const EXCEPTION_MISSING_ENDPOINT = "The endpoint you've tried to access does not exist. Check your URL.";
+    const EXCEPTION_MISSING_ENDPOINT = "The endpoint you've tried to access does not exist. Check if the domain matches the domain you have configure on Mailgun.";
     const TOO_MANY_RECIPIENTS = "You've exceeded the maximum recipient count (1,000) on the to field with autosend disabled.";
     const INVALID_PARAMETER_NON_ARRAY = "The parameter you've passed in position 2 must be an array.";
     const INVALID_PARAMETER_ATTACHMENT = 'Attachments must be passed with an "@" preceding the file path. Web resources not supported.';
diff --git a/src/Mailgun/Exception/HttpClientException.php b/src/Mailgun/Exception/HttpClientException.php
index 140334b..1c65951 100644
--- a/src/Mailgun/Exception/HttpClientException.php
+++ b/src/Mailgun/Exception/HttpClientException.php
@@ -70,7 +70,7 @@ final class HttpClientException extends \RuntimeException implements Exception
 
     public static function notFound(ResponseInterface $response = null)
     {
-        return new self('The endpoint you tried to access does not exist. Check your URL.', 404, $response);
+        return new self('The endpoint you have tried to access does not exist. Check if the domain matches the domain you have configure on Mailgun.', 404, $response);
     }
 
     /**