Hi, if the eventID is "Fail", then the messages are unrecoverable and not delivered and they would have bounced back to the sender.
The RecipientStatus will tell you why they failed.
Get-MessageTrackingLog -ResultSize Unlimited -Start "01/28/2023 00:00:00" -End "01/28/2023 23:59:59" -EventId "Fail" | FL RecipientStatus