email validation problem

Imports System.Net.Mail
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
'Declare variables
Dim sSelectedSMTPServer As String = ""
Dim iSelectedSMTPPort As Integer

'Check wich email account is selected and provide correct smtp address / port.
Select Case True
Case CheckBox1.Checked
sSelectedSMTPServer = "smtp.gmail.com"
iSelectedSMTPPort = 587
Case CheckBox2.Checked
sSelectedSMTPServer = "smtp.live.com"
iSelectedSMTPPort = 587
Case CheckBox3.Checked
sSelectedSMTPServer = "smtp.mail.yahoo.com"
iSelectedSMTPPort = 465
Case Else
'If no account is selected you will get a message and the program will not attempt to send the email.
MsgBox("Please choose your email service.", vbInformation)
End Select
'For multiple selection
If CheckBox1.Checked And CheckBox2.Checked Then
MsgBox("You must select only one email service.", vbInformation)
ElseIf CheckBox1.Checked And CheckBox3.Checked Then
MsgBox("You must select only one email service.", vbInformation)
ElseIf CheckBox2.Checked And CheckBox3.Checked Then
MsgBox("You must select only one email service.", vbInformation)
ElseIf CheckBox1.Checked And CheckBox2.Checked And CheckBox3.Checked Then
MsgBox("You must select only one email service.", vbInformation)
End If

Exit Sub

Dim SMTP As New SmtpClient
'SSL enabled for outgoing mail.
SMTP.EnableSsl = True
'Get login credentials.
SMTP.Credentials = New System.Net.NetworkCredential(TextBox1.Text, TextBox2.Text)
'SMTP Port that will be used.
SMTP.Port = iSelectedSMTPPort

Try
'What???
MsgBox("Successfully logged", MsgBoxStyle.Information, "Report")
Me.Close()
Catch ex As Exception
MsgBox("Unsuccessfully logged", MsgBoxStyle.Exclamation, "Report")
End Try
End Sub