diff --git a/src/SMTP.php b/src/SMTP.php
index 5f6f8b17..8411b476 100644
--- a/src/SMTP.php
+++ b/src/SMTP.php
@@ -146,11 +146,13 @@ class SMTP
     /**
      * @var array Patterns to extract an SMTP transaction id from reply to a DATA command.
      * The first capture group in each regex will be used as the ID.
+     * MS ESMTP returns the message ID, which may not be correct for internal tracking.
      */
     protected $smtp_transaction_id_patterns = [
         'exim' => '/[0-9]{3} OK id=(.*)/',
         'sendmail' => '/[0-9]{3} 2.0.0 (.*) Message/',
-        'postfix' => '/[0-9]{3} 2.0.0 Ok: queued as (.*)/'
+        'postfix' => '/[0-9]{3} 2.0.0 Ok: queued as (.*)/',
+        'Microsoft_ESMTP' => '/[0-9]{3} 2.[0-9].0 (.*)@(?:.*) Queued mail for delivery/'
     ];
 
     /**