Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Try
      Dim client As New SmtpClient()
...
      Dim message As New MailMessage()
...
      AddHandler client.SendCompleted, AddressOf SendCompletedCallback
      client.SendAsync(message, "userToken")
    Catch
      MsgBox(Err.Description)
    End Try
  End Sub

  Private Sub SendCompletedCallback(ByVal sender As Object, ByVal e As AsyncCompletedEventArgs)
    ' Get the unique identifier for this asynchronous operation. 
    Dim token As String = CStr(e.UserState)

    If e.Cancelled Then
      MsgBox("Send canceled.")
    End If
    If e.Error IsNot Nothing Then
      MsgBox(e.Error.Message, , token)
    Else
      MsgBox("Message sent.")
    End If
  End Sub

Public Function SendMailMessage(Optional ByVal m_msg_FromAddress As String = vbNullString,
...
                                  Optional ByVal m_CallbackFunction As [Delegate] = Nothing,
                                  Optional ByVal m_emailToken As String = vbNullString,
                                  Optional ByRef str_error As String = vbNullString
                                  ) As Boolean

          If IsNothing(m_CallbackFunction) Then
            AddLogCDO(Now.ToString & Chr(9) & "Sending mail")
            .Send(message)
            AddLogCDO(Now.ToString & Chr(9) & "Sending mail...Done")
            AddLogCDO("-------------------------")
            Return True
          Else
            AddLogCDO(Now.ToString & Chr(9) & "Sending mail asynchronously")
            AddLogCDO(Chr(9) & Chr(9) & "Token=" & m_emailToken)
            AddHandler client.SendCompleted, m_CallbackFunction
            .SendAsync(message, m_emailToken)
            AddLogCDO(Now.ToString & Chr(9) & "Sending mail asynchronously...waiting for the result")
            AddLogCDO("-------------------------")
            Return True
          End If
...
    If SendMailMessage("sender@hotmail.com",...
     ..."smtp.live.com", 567, True, "sender@hotmail.com", "password", True, False, AddressOf SendCompletedCallback, , str_err) ...

Public Function SendMailMessage(...
...
            AddHandler client.SendCompleted, AddressOf SendCompletedCallback
            .SendAsync(message, m_emailToken)
            AddLogCDO(Now.ToString & Chr(9) & "Sending mail asynchronously...waiting for the result")
            Do
               If (есть результат в SendCompletedCallback) Then Exit Do
               Application.DoEvents() 'чтоб она не "вешала" форму, и именно поэтому .Send(message) не катит
            Loop
            return true/[false + str_error ]