@neil I'd then say if the email was delivered, i.e. a 2xx response from one of the recipient's domain's MX record, then it has in fact been delivered.
Getting it reliably from that point to the attention of an actual person or process is the recipients responsibility.
There is always a technical grey area catch on such things but that is a massive edge case. Far more likely email logs shows delivery attempt (and non delivery response) or delivery response.
But the border is the recipient MX.