Email med asp.net (vb)

Exception Details: System.Runtime.InteropServices.COMException: The server rejected the sender address. The server response was: 501 5.5.4 Invalid Address

Source Error:


Line 121:        mail.BodyFormat = 1
Line 122:        SmtpMail.SmtpServer = "Localhost"
Line 123:        SmtpMail.Send(mail)
Line 124:    End Sub
Line 125:   

Selve koden jeg bruger er

Sub SendEmail
        Dim mail As New MailMessage
        mail.From = "Automated Email System"
        mail.To = txtEmail.Text
        mail.Subject = "Confirmation of your Username and Password"
        mail.Body = "Your username and password is" & txtUsername.text & txtPassword.text
    mail.BodyFormat = 1
    SmtpMail.SmtpServer = "Localhost"
    SmtpMail.Send(mail)
End Sub

er det en fejl i min kode eller manglende opsætning af min smtp server på win 2003 server?