diff --git a/src/Symfony/Component/Mailer/Bridge/Postmark/Transport/PostmarkApiTransport.php b/src/Symfony/Component/Mailer/Bridge/Postmark/Transport/PostmarkApiTransport.php
index ea5ac37671c12..1ed5e5c6bc644 100644
--- a/src/Symfony/Component/Mailer/Bridge/Postmark/Transport/PostmarkApiTransport.php
+++ b/src/Symfony/Component/Mailer/Bridge/Postmark/Transport/PostmarkApiTransport.php
@@ -147,7 +147,7 @@ private function getAttachments(Email $email): array
             ];
 
             if ('inline' === $disposition) {
-                $att['ContentID'] = 'cid:'.$filename;
+                $att['ContentID'] = 'cid:'.($attachment->hasContentId() ? $attachment->getContentId() : $filename);
             }
 
             $attachments[] = $att;