C# - Changing the Return-Path for MailItem

Question

I have a process that will sit on an Automated Emailing machine to send emails based on a flat file. The file provides the To, CC, Subject, Body, Attachments, and Sender. When the email is sent, I want any failed emails or bounce backs to go to the sender rather than the automated service. I have tried two solutions that work...partially.

First using SMTP, I am able to set the Return-Path and the From addresses and get bounced emails to go back to the sender. However, company policy is that I can not hard code user-ids and/or passwords. I have tried to use the "UseDefaultCredentials" flag with no luck. Our AD login and Exchange login are different and the password is the same. Here is the code:

    private void SendEmail(EmailProperties pEmail)
    {
        try
        {
            MailMessage mail = new MailMessage();
            mail.Headers.Add("Return-Path", AddressLookup(pEmail.Sender));
            foreach (string strTo in pEmail.To)
            {
                mail.To.Add(AddressLookup(strTo));
            }
            foreach (string strCC in pEmail.CC)
            {
                mail.CC.Add(AddressLookup(strCC));
            }
            mail.Subject = pEmail.Subject;
            mail.Body = pEmail.Body;
            mail.From = new MailAddress(AddressLookup(pEmail.Sender));
            //mail.ReplyToList.Add(AddressLookup(pEmail.ReplyTo));
            SmtpClient smtpClient = new SmtpClient("server", 25);
            smtpClient.DeliveryMethod = SmtpDeliveryMethod.Network;
            smtpClient.UseDefaultCredentials = true;
            //smtpClient.Credentials = new NetworkCredential("user", "pw");
            smtpClient.EnableSsl = false;
            smtpClient.Send(mail);
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }
    private string AddressLookup(string pstrAddr)
    {
        try
        {
            if (pstrAddr.Contains("@")) return pstrAddr;

            Outlook.Application OutlookApp = new Outlook.Application();
            Outlook.AddressLists addrLists = OutlookApp.Session.AddressLists;
            Outlook.AddressList gal = addrLists["Global Address List"];
            Outlook.AddressEntry entry = gal.AddressEntries[pstrAddr];
            return entry.GetExchangeUser().PrimarySmtpAddress;
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

Second: I have tried using the Outlook Interop and I am able to send the email fine but any bounce backs go the the automated service. Here is the Code:

    private void SendEmail(EmailProperties pEmail)
    {
        try
        {
            Outlook.Application OutlookApp = new Outlook.Application();
            Outlook.MailItem mail = OutlookApp.CreateItem(
                Outlook.OlItemType.olMailItem) as Outlook.MailItem;
            Outlook.AddressEntry currentUser = 
            OutlookApp.Session.CurrentUser.AddressEntry;
            if (currentUser.Type == "EX")
            {
                mail.To = pEmail.To;
                mail.CC = pEmail.CC;
                mail.Subject = pEmail.Subject;
                mail.Body = pEmail.Body;
                mail.ReplyRecipients.Add(pEmail.ReplyTo);
                mail.Sender = AddressLookup(pEmail.Sender);
                mail.Recipients.ResolveAll();
                mail.ReplyRecipients.ResolveAll();

                foreach (string e in pEmail.Attachments)
                {
                    mail.Attachments.Add(e, Outlook.OlAttachmentType.olByValue,
                        Type.Missing, Type.Missing);
                }
                mail.Send();
            }
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

Is there something I am missing or another direction I can go with this. I would prefer to use the interop to send the emails and just change the email header. If there is a better way of doing this, I am open to suggestions.

Thanks!