You may experienced that ColdFusion (all versions) from time to times does not deliver all mails and moves some mails to Mail\Undelivr folder. These mails stay there and will never delivered. Normally this behavior is correct for emails with invalid email addresses, but in many many cases this happens only because ColdFusion temporarily cannot reach your mail gateway. The root cause here is ColdFusion that does not retry delivery properly like any other properly working email system.