Error message user authentication error 5.5.1.while sending mail through asp using smtp server

Imports System.Net.Mail
Imports System.Net

Partial Class Contact
    Inherits System.Web.UI.Page
    Public Property Credentials As ICredentialsByHost

    Protected Sub ok_Click(sender As Object, e As EventArgs) Handles ok.Click

            Dim fromaddr As String = txtemail.Text
            Dim toaddr As String = "Email Address"
        Dim password As String = "Password"

            Dim msg As MailMessage
            Dim smtp As SmtpClient
            Dim nc As NetworkCredential
            msg = New MailMessage()

            msg.Subject = lname.Text
            msg.From = New MailAddress(fromaddr)
            msg.Body = txtquery.Text
            msg.To.Add(New MailAddress(txtemail.Text))
            smtp = New SmtpClient()
            smtp.Credentials = New System.Net.NetworkCredential(fromaddr, password)
        smtp.Host = "smtp.gmail.com"
            smtp.Port = 587
        smtp.UseDefaultCredentials = False
            smtp.EnableSsl = True
        smtp.Send(msg)
        Try
            smtp.Send(msg)
            MsgBox("email sent")
        Catch ex As Exception
            Response.Write("<script>alert('" + ex.ToString() + "');</script>")
        End Try
    End Sub

End Class