We do not monitor/log emails, we monitor/log SMTP Connections. This is entirely dependant on the sending domain, and whether the message was sent in one SMTP connection or was split into several smtp connections.
In the analytics the message detail is taken from the headers of the email and not the SMTP Log. This is why you will see entries for emails that was not destined based on the SMTP connection.
Typically you will see a message like the following, in Analytics --> Message Details --> and hover over a greyed out recipient:
The recipient may be greyed out, and you can see the message "No SMTP Transport - Recipient was internal or skipped". This does not mean the email wasn't delivered, but rather means the email needed to be split into 2 or more SMTP connections and delivered on a different SMTP connections.
You can see this by running a report on the Subject of the email in question, and checking the Date, Sender and Recipients match almost exactly. There may be a few seconds difference between one log and another, due to splitting the connection and then delivery time.
You will notice there may be additional logs, and will find that depending on the log different Recipients are highlighted depending on the SMTP Connection and how it was split.
The screenshots highlight the different perspectives depending on how the SMTP connection was split.
However, if you see that the recipients are internal, there may not be a log of this, as depending on your configured routing these should be delivered without being logged or processed by our system.
If you do not see a recipient that got the email then this is was down to the sending server not sending to that recipient for any number of reasons. The customer needs to contact the remote domain to see why the email was not sent to that recipient.