The meg device uses the 821 address in the reports. If the message is missing in the reports, but the headers indicate that the message went through the device, then its most likely that the sending address you see in the "From:" field is different than the actual 821 email address used. You might want to try searching by subject.
Another possibility could be that this mail has not been subject to any filtering and went straight thru the appliance as legitimate mail AND at the same time a certain logging event type was not enabled (which is not enabled by default) and thus this email did not count among the number of Delivered emails in the PostGres database.
If you cannot query the Legitimate emails - which is the case when you do not have a scanner type LG recorded in PostGres reporting database then this could be the cause.