How to send mail using ASP through proxy server?

My computer is at intranet,I want to use following code to realizing
function of sending mail.But this program only can send mail between
intranet.If I send mail in internet,I need to through proxy server,the proxy
method is HTTP(proxy server:xx.yyyy.com,port:8080,username/password),I want
to use this program to send from intranet to internet through proxy server.I
want to know how to realize this function.My code is follows:
Set objCDOMail = Server.CreateObject("CDONTS.NewMail")
objCDOMail.From = frommail
objCDOMail.To = tomail
objCDOMail.Subject = mailsubject
objCDOMail.BodyFormat = 0
objCDOMail.MailFormat = 0
objCDOMail.Body = mailbody
objCDOMail.Send
Set objCDOMail = Nothing

Any idea will be appreciated!
Thanks in advance!