diff --git a/redaxo/src/addons/phpmailer/lib/mailer.php b/redaxo/src/addons/phpmailer/lib/mailer.php
index 42114b3d47..cf22c12b3c 100644
--- a/redaxo/src/addons/phpmailer/lib/mailer.php
+++ b/redaxo/src/addons/phpmailer/lib/mailer.php
@@ -382,12 +382,31 @@ protected function microsoft365Send(): bool
         }
 
         $customHeaders = [];
+        $extendedProperties = [];
         /** @var array{string, string} $header */
         foreach ($this->getCustomHeaders() as $header) {
-            $customHeaders[] = [
-                'name' => $header[0],
-                'value' => $this->encodeHeader(trim($header[1])),
-            ];
+            $name = trim($header[0]);
+            $value = trim($header[1]);
+
+            if (str_starts_with(strtolower($name), 'x-')) {
+                $customHeaders[] = [
+                    'name' => $name,
+                    'value' => $this->encodeHeader($value),
+                ];
+                continue;
+            }
+
+            if ('list-unsubscribe' === strtolower($name)) {
+                // MAPI property PidTagListUnsubscribe; Exchange emits it as List-Unsubscribe header
+                $extendedProperties[] = [
+                    'id' => 'String 0x1045',
+                    'value' => $value,
+                ];
+                continue;
+            }
+
+            // The Graph API rejects the whole request for custom headers without "x-" prefix,
+            // so headers without a MAPI mapping (e.g. Auto-Submitted) have to be dropped
         }
 
         // Attachments für Graph API aufbereiten
@@ -470,6 +489,9 @@ protected function microsoft365Send(): bool
         if (!empty($customHeaders)) {
             $mailData['message']['internetMessageHeaders'] = $customHeaders;
         }
+        if (!empty($extendedProperties)) {
+            $mailData['message']['singleValueExtendedProperties'] = $extendedProperties;
+        }
         if ('' !== $this->ConfirmReadingTo) {
             // MS Graph API unterstützt keine Read-Receipts an beliebige Empfänger
             $mailData['message']['isReadReceiptRequested'] = true;