Ho un errore per questo programma per inviare mail
ho provato a usare si out.alice.it che mail.gmail.com ma non funziona come mai ?

codice:
Imports System.Net
Imports System.Net.Mail
Public Class Form5
    Dim openfile As New OpenFileDialog
    Dim posta As New MailMessage()
    Private Sub Panel1_Paint(sender As System.Object, e As System.Windows.Forms.PaintEventArgs) Handles Panel1.Paint

    End Sub


    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click

        posta = New MailMessage
        posta.From = New MailAddress(TextBox4.Text)
        posta.To.Add("andrea.moro1989@gmail.com")
        posta.Subject = TextBox2.Text
        posta.Priority = MailPriority.High
        posta.Body = RichTextBox1.Text
        ' If TextBox4.Text = "" Then
        ' Else
        'posta.Attachments.Add(New Attachment(TextBox3.Text))
        'End If

        Dim smtp As New SmtpClient(ComboBox1.Text)
        smtp.Send(posta)
        MsgBox("La Mail è Stata Inviata!", MsgBoxStyle.Information, "Mail")
    End Sub

   

    Private Sub Label3_Click(sender As System.Object, e As System.EventArgs) Handles Label3.Click

    End Sub

    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
        openfile.ShowDialog()
        Try
            TextBox3.Text = openfile.FileName
        Catch ex As Exception
        End Try

    End Sub

    Private Sub TextBox4_TextChanged(sender As System.Object, e As System.EventArgs) Handles TextBox4.TextChanged

    End Sub

    Private Sub Form5_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        ComboBox1.Items.Add("out.alice.it")
        ComboBox1.Items.Add("mail.gmail.com")
        ComboBox1.Items.Add("out.aliceposta.it")
        ComboBox1.Items.Add("smtp.teletu.it")
        ComboBox1.Items.Add("smtp.net.vodafone.it")
    End Sub
End Class