Any chance you can advise me as to why if I only popular AddMultipleBcc that it does not place anything in the To header field?
The difference between BCC and CC is that the CC email addresses are included in the email’s MIME (in the "CC" header field).
BCC addresses do not appear in the MIME source of the email, hence the recipient cannot see the BCC list. The BCC addresses are, however, passed in "RCPT TO" commands to the SMTP server when sending the email. (and so are the To and CC addresses.)