diff --git a/smtp.mail.inc b/smtp.mail.inc
index b01ba00..e64dd37 100644
--- a/smtp.mail.inc
+++ b/smtp.mail.inc
@@ -271,6 +271,10 @@ class SmtpMailSystem implements MailSystemInterface {
           }
           break;

+        case 'message-id':
+          $mailer->MessageID = $value;
+          break;
+
         default:
           // The header key is not special - add it as is.
           $mailer->AddCustomHeader($key . ': ' . $value);