用VB实现一个简单的ESMTP客户端

p;msgsend = "DATA" + FLAG_LINE_END
            smtpClient.SendData msgsend
            Text1.Text = Text1.Text + msgsend + FLAG_LINE_END
            state = 5
        Case 550, 551, 552, 553, 450, 451, 452    ''failed
                Quit
                state = 7

        Case 500, 501, 503, 421 ''error
            Quit
            state = 7
        End Select
    Case 5  ''DATA been sent
        Select Case msgType
        Case 354
            Send "from", "to", "no subject", "plain", "test"
            Text1.Text = Text1.Text + msgsend + FLAG_LINE_END
            state = 6
        Case 451, 554
        Case 500, 501, 503, 421
        End Select
    Case 6  ''body been sent
        Select Case msgType
        Case 250
                Quit
                state = 7
        Case 552, 451, 452
        Case 500, 501, 502, 421
        End Select
    Case 7
        Select Case msgType
        Case 221    ''process disconnected
            state = 0
        Case 500    ''command error
        End Select
    End Select
    
End Sub

Private Sub Quit()
    Dim msgsend As String
    rs.Close
    conn.Close
    msgsend = "QUIT" + FLAG_LINE_END
   &nb