IMultipartReportReportType Property

string ReportType { get; set; }

public void ProcessDeliveryStatusNotification (MimeMessage message)
{
    var report = message.Body as MultipartReport;

    if (report == null || report.ReportType == null || !report.ReportType.Equals ("delivery-status", StringComparison.OrdinalIgnoreCase)) {
        // this is not a delivery status notification message...
        return;
    }

    // process the report
    foreach (var mds in report.OfType<MessageDeliveryStatus> ()) {
        // process the status groups - each status group represents a different recipient

        // The first status group contains information about the message
        var envelopeId = mds.StatusGroups[0]["Original-Envelope-Id"];

        // all of the other status groups contain per-recipient information
        for (int i = 1; i < mds.StatusGroups.Length; i++) {
            var recipient = mds.StatusGroups[i]["Original-Recipient"];
            var action = mds.StatusGroups[i]["Action"];

            if (recipient == null)
                recipient = mds.StatusGroups[i]["Final-Recipient"];

            // the recipient string should be in the form: "rfc822;user@domain.com"
            var index = recipient.IndexOf (';');
            var address = recipient.Substring (index + 1);

            switch (action) {
            case "failed":
                Console.WriteLine ("Delivery of message {0} failed for {1}", envelopeId, address);
                break;
            case "delayed":
                Console.WriteLine ("Delivery of message {0} has been delayed for {1}", envelopeId, address);
                break;
            case "delivered":
                Console.WriteLine ("Delivery of message {0} has been delivered to {1}", envelopeId, address);
                break;
            case "relayed":
                Console.WriteLine ("Delivery of message {0} has been relayed for {1}", envelopeId, address);
                break;
            case "expanded":
                Console.WriteLine ("Delivery of message {0} has been delivered to {1} and relayed to the the expanded recipients", envelopeId, address);
                break;
            }
        }
    }
}