diff --git a/src/SMTP.php b/src/SMTP.php
index 1b5b00771..aa7ccfcb0 100644
--- a/src/SMTP.php
+++ b/src/SMTP.php
@@ -187,12 +187,12 @@ class SMTP
      */
     protected $smtp_transaction_id_patterns = [
         'exim' => '/[\d]{3} OK id=(.*)/',
-        'sendmail' => '/[\d]{3} 2.0.0 (.*) Message/',
-        'postfix' => '/[\d]{3} 2.0.0 Ok: queued as (.*)/',
-        'Microsoft_ESMTP' => '/[0-9]{3} 2.[\d].0 (.*)@(?:.*) Queued mail for delivery/',
+        'sendmail' => '/[\d]{3} 2\.0\.0 (.*) Message/',
+        'postfix' => '/[\d]{3} 2\.0\.0 Ok: queued as (.*)/',
+        'Microsoft_ESMTP' => '/[0-9]{3} 2\.[\d]\.0 (.*)@(?:.*) Queued mail for delivery/',
         'Amazon_SES' => '/[\d]{3} Ok (.*)/',
         'SendGrid' => '/[\d]{3} Ok: queued as (.*)/',
-        'CampaignMonitor' => '/[\d]{3} 2.0.0 OK:([a-zA-Z\d]{48})/',
+        'CampaignMonitor' => '/[\d]{3} 2\.0\.0 OK:([a-zA-Z\d]{48})/',
         'Haraka' => '/[\d]{3} Message Queued \((.*)\)/',
         'ZoneMTA' => '/[\d]{3} Message queued as (.*)/',
         'Mailjet' => '/[\d]{3} OK queued as (.*)/',