I believe you'll need to use the outlook API's to perform that, since the MailObject and SMTP will send the mail internally using the mentioned params, something like this should help http://www.codeproject.com/Tips/165548/C-Code-snippet-to-send-an-Email-with-attachment-fr
A possible duplicate : Can only send email via Outlook if Outlook is openCan only send email via Outlook if Outlook is open