An email was relayed by HES successfully and also reached the end user's mailbox but it is not showing in the HES Message Tracking log.
Usually, normal mails have different message IDs. But the mail in question has the exact same Sender, Recipient, Timestamp and Message-ID with another mail.
So when the customer queries the Message Tracking log, the HES UI will get multiple exact same results. In such scenario, HES will treat these results as duplicate log entries and only show one of them.
This is a product spec.
Such issue is usually caused by pressure tests with certain mail test tools. Therefore, it is recommended that customers should not test the HES system with such tools.