This is a C# source code that reads a signed MDN.  The complete program can be downloaded at EDIINT AS2. 
A detailed explanation of the algorithm can be read at "Creating an AS2 client in stages".

private void ReadMdnSignedMessage(ref mailMessage oMDN)
        {
            // Algotrithm to read the message in the MDN
            mailMessages oMdnMsgs = oMDN.GetMessages();
            int nMdnResult = -1;
            string sErrorMsg = "";
            string sRecvMicFldValue = "";
            string sMicValue = "";
            string sMicAlg = "";

            for (int ii = 1; ii <= oMdnMsgs.Count; ii++)
            {

                mailMessage oMdnMsg = oMdnMsgs.GetMessageContent(ii);
                mailContentType oContentType = oMdnMsg.GetContentType();

                if (oContentType.MediaType == "message" && oContentType.SubType == "disposition-notification")
                {
                    mailMessages oSubMessages = oMdnMsg.GetMessages();
                    mailMessage oDisposition = oSubMessages.GetMessageContent(1);

                    if (oDisposition != null)
                    {

                        mailHeaders oHeaders = oDisposition.GetHeaders();
                        string sHeaders = "";

                        for (int i = 1; i <= oHeaders.Count; i++)
                        {
                            mailHeader oHeader = oHeaders.GetHeaderByIndex(i);
                            sHeaders = sHeaders + oHeader.Name + " : " + oHeader.Value + "\r\n";
                            if (oHeader.Name.ToLower() == "disposition")
                            {
                                nMdnResult = 0;
                                //MessageBox.Show oHeader.Value
                            }
                            else if (oHeader.Name.ToLower() == "warning")
                            {
                                nMdnResult = 1;
                                sErrorMsg = sErrorMsg + oHeader.Value + "\r\n";
                            }
                            else if (oHeader.Name.ToLower() == "error")
                            {
                                nMdnResult = 2;
                                sErrorMsg = sErrorMsg + oHeader.Value + "\r\n";
                            }

                            if (oHeader.Name == "Received-Content-MIC")
                            {
                                sRecvMicFldValue = oHeader.Value;

                                int lPos;
                                lPos = sRecvMicFldValue.IndexOf(",");
                                if (lPos == 0 && sRecvMicFldValue.Length > 0)
                                {
                                    nMdnResult = 3;
                                }
                                else
                                {
                                    sMicValue = sRecvMicFldValue.Substring(0, lPos);
                                    sMicAlg = sRecvMicFldValue.Substring(lPos + 1);

                                    // Compare the MIC of the subject document to MIC received by received.
                                    string sSubjMicValue = txtMIC.Text;
                                    if (sMicValue != sSubjMicValue)
                                    {
                                        nMdnResult = 4;
                                    }

                                }   //lPos == 0 && Len(sRecvMicFldValue) > 0

                            }

                        }    // For Each oHeader In oHeaders

                    }   // if (oDisposition)

                }   // if (oContentType.MediaType && oContentType.SubType)

            }    // for ii

            if (nMdnResult == 0)
            {
                MessageBox.Show("Successfully sent.", "MDN Message");
            }
            else if (nMdnResult == 1)
            {
                MessageBox.Show("The message was successfully sent but there were warnings.");
                MessageBox.Show(sErrorMsg, "MDN Warning");
            }
            else if (nMdnResult == 2)
            {
                MessageBox.Show("An error occurred during transmission");
                MessageBox.Show(sErrorMsg, "MDN Error");
            }
            else if (nMdnResult == 3)
            {
                MessageBox.Show("Invalid Received-Content-MIC field value");

            }
            else if (nMdnResult == 4)
            {
                MessageBox.Show("Recipient received invalid document. Subject MIC=" + txtMIC.Text + ", Recv MIC=" + sMicValue);

            }   
            else
            {
                MessageBox.Show("MDN format not recognized");

            } //if (nMdnResult) 

        }        
        

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