what is the meaning of this error in Asp.net ?

i developed one program for sending mail in asp.net .. when i execute my program from local server ,it shows the below the error.

net.io.connectionclosed.. how i can correct this error.. This is the code for my mail application..

 Imports System.Net.Mail.SmtpClient
Partial Class PhaseEmail
    Inherits System.Web.UI.Page
 
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
 
    End Sub
 
    Protected Sub BtnSend_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BtnSend.Click
        Dim MailObj As New System.Net.Mail.SmtpClient
        MailObj.Host = "localhost"
        MailObj.Send(TxtFrom.Text, TxtTo.Text, TxtSubject.Text, TxtMessage.Text)
        Label1.Text = "Message sent successfully"
    End Sub
End Class