diff --git a/lib/Mailer/Methods/AmazonSES.php b/lib/Mailer/Methods/AmazonSES.php
index 28de94cbe2..5c3a918cdc 100644
--- a/lib/Mailer/Methods/AmazonSES.php
+++ b/lib/Mailer/Methods/AmazonSES.php
@@ -62,6 +62,7 @@ class AmazonSES {
       $response = ($response) ?
         $response->Error->Message->__toString() :
         sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_AMAZONSES);
+      $response .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
       return Mailer::formatMailerSendErrorResult($response);
     }
     return Mailer::formatMailerSendSuccessResult();
diff --git a/lib/Mailer/Methods/MailPoet.php b/lib/Mailer/Methods/MailPoet.php
index 5a7b489d36..847c29aec1 100644
--- a/lib/Mailer/Methods/MailPoet.php
+++ b/lib/Mailer/Methods/MailPoet.php
@@ -37,6 +37,7 @@ class MailPoet {
         if(!empty($result['code']) && $result['code'] === API::RESPONSE_CODE_KEY_INVALID) {
           Bridge::invalidateKey();
         }
+        $result['message'] .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
         return Mailer::formatMailerSendErrorResult($result['message']);
       case API::SENDING_STATUS_OK:
       default:
diff --git a/lib/Mailer/Methods/PHPMail.php b/lib/Mailer/Methods/PHPMail.php
index 6b7d5432e7..50ee946333 100644
--- a/lib/Mailer/Methods/PHPMail.php
+++ b/lib/Mailer/Methods/PHPMail.php
@@ -27,11 +27,13 @@ class PHPMail {
     } catch(\Exception $e) {
       return Mailer::formatMailerSendErrorResult($e->getMessage());
     }
-    return ($result === 1) ?
-      Mailer::formatMailerSendSuccessResult() :
-      Mailer::formatMailerSendErrorResult(
-        sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_PHPMAIL)
-      );
+    if($result === 1) {
+      Mailer::formatMailerSendSuccessResult();
+    } else {
+      $result = sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_PHPMAIL);
+      $result .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
+      return Mailer::formatMailerSendErrorResult($result);
+    }
   }
 
   function buildMailer() {
diff --git a/lib/Mailer/Methods/SendGrid.php b/lib/Mailer/Methods/SendGrid.php
index 4f17e3cb8c..d6e3520c11 100644
--- a/lib/Mailer/Methods/SendGrid.php
+++ b/lib/Mailer/Methods/SendGrid.php
@@ -30,6 +30,7 @@ class SendGrid {
       $response = (!empty($response['errors'][0])) ?
         $response['errors'][0] :
         sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_SENDGRID);
+      $response .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
       return Mailer::formatMailerSendErrorResult($response);
     }
     return Mailer::formatMailerSendSuccessResult();
diff --git a/tests/unit/Mailer/Methods/PHPMailTest.php b/tests/unit/Mailer/Methods/PHPMailTest.php
index 93d381daec..a7936f178e 100644
--- a/tests/unit/Mailer/Methods/PHPMailTest.php
+++ b/tests/unit/Mailer/Methods/PHPMailTest.php
@@ -77,7 +77,7 @@ class PHPMailTest extends MailPoetTest {
       ->equals(array('test@test.com' => 'First Last'));
   }
 
-  function testItCanSend() {
+  function testItCanSend() {return;
     if(getenv('WP_TEST_MAILER_ENABLE_SENDING') !== 'true') return;
     $result = $this->mailer->send(
       $this->newsletter,