diff --git a/lib/Mailer/Methods/AmazonSES.php b/lib/Mailer/Methods/AmazonSES.php
index 5c3a918cdc..be4160a5ce 100644
--- a/lib/Mailer/Methods/AmazonSES.php
+++ b/lib/Mailer/Methods/AmazonSES.php
@@ -62,7 +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);
+      $response .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), htmlspecialchars($subscriber));
       return Mailer::formatMailerSendErrorResult($response);
     }
     return Mailer::formatMailerSendSuccessResult();
diff --git a/lib/Mailer/Methods/MailPoet.php b/lib/Mailer/Methods/MailPoet.php
index 847c29aec1..e08fc48cdd 100644
--- a/lib/Mailer/Methods/MailPoet.php
+++ b/lib/Mailer/Methods/MailPoet.php
@@ -37,7 +37,7 @@ class MailPoet {
         if(!empty($result['code']) && $result['code'] === API::RESPONSE_CODE_KEY_INVALID) {
           Bridge::invalidateKey();
         }
-        $result['message'] .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
+        $result['message'] .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), htmlspecialchars($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 50ee946333..b3cf01df2a 100644
--- a/lib/Mailer/Methods/PHPMail.php
+++ b/lib/Mailer/Methods/PHPMail.php
@@ -31,7 +31,7 @@ class PHPMail {
       Mailer::formatMailerSendSuccessResult();
     } else {
       $result = sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_PHPMAIL);
-      $result .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
+      $result .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), htmlspecialchars($subscriber));
       return Mailer::formatMailerSendErrorResult($result);
     }
   }
diff --git a/lib/Mailer/Methods/SMTP.php b/lib/Mailer/Methods/SMTP.php
index 795175e696..86f20c2c61 100644
--- a/lib/Mailer/Methods/SMTP.php
+++ b/lib/Mailer/Methods/SMTP.php
@@ -116,7 +116,7 @@ class SMTP {
     } else {
       $message = sprintf(__('%s has returned an unknown error.', 'mailpoet'), Mailer::METHOD_SMTP);
     }
-    $message .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), $subscriber);
+    $message .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), htmlspecialchars($subscriber));
     return $message;
   }
 
diff --git a/lib/Mailer/Methods/SendGrid.php b/lib/Mailer/Methods/SendGrid.php
index d6e3520c11..e0b5b56cd9 100644
--- a/lib/Mailer/Methods/SendGrid.php
+++ b/lib/Mailer/Methods/SendGrid.php
@@ -30,7 +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);
+      $response .= sprintf(' %s: %s', __('Unprocessed subscriber', 'mailpoet'), htmlspecialchars($subscriber));
       return Mailer::formatMailerSendErrorResult($response);
     }
     return Mailer::formatMailerSendSuccessResult();