diff --git a/lib/phpmailer/moodle_phpmailer.php b/lib/phpmailer/moodle_phpmailer.php
index b502d4a22a3..b3955ae5a89 100644
--- a/lib/phpmailer/moodle_phpmailer.php
+++ b/lib/phpmailer/moodle_phpmailer.php
@@ -52,6 +52,8 @@ class moodle_phpmailer extends PHPMailer {
         global $CFG;
         $this->Version   = 'Moodle '.$CFG->version;         // mailer version
         $this->CharSet   = 'UTF-8';
+        // MDL-52637: Disable the automatic TLS encryption added in v5.2.10 (9da56fc1328a72aa124b35b738966315c41ef5c6).
+        $this->SMTPAutoTLS = false;
 
         if (!empty($CFG->smtpauthtype)) {
             $this->AuthType = $CFG->smtpauthtype;