Error: SMTPsvg.Mailer

I'm using a block of ASP to allow a user to send a form via e-mail. However, someone keeps sending me spam through this form and they're using a bogus return address. I'm testing for a successful send, which should fail if the return address is not valid, but I'm still getting the junk.

The block looks like this:

Set Mailer = Server.CreateObject("SMTPsvg.Mailer")
Mailer.RemoteHost = "smtp.xxx.com"
Mailer.FromName = Request.QueryString ("Name")
Mailer.FromAddress = Request.QueryString ("Email")
Mailer.AddRecipient "Web Mail", "PCT2@xxx.com"
Mailer.Subject = "P.C.T. E-mail"
Mailer.BodyText = UserString
if Mailer.SendMail then
Response.Write " - Sucessful - "
else
Response.Write " - Failed - "
Response.Write Mailer.Response
end if

Should this block be stopping bogus From addresses? Or do I need to be doing something different?