System.net.mail can send email when upload to webhost but it doesn't work in localhost

Dear All, as mentioned in the post subject, system.net.mail doesn't work when it runs in localhost. Appreciate if you guys can enlighten me.

The following is my code:-

Imports System.Data.OleDb
Imports System.Data
Imports System.Net.Mail
Imports System.IO

'email user the activation code

Dim email_admin, email_user As String
Dim mailsubject As String
Dim mailbody As String

Dim email_user_message As String

email_user_message = "my email message"

email_user = txt_email.Text
email_admin = "abcd@xxx.com"

mailsubject = "my mail subject"

Dim MailMessage As New MailMessage()
MailMessage.To.Add(email_user)
MailMessage.Bcc.Add(email_admin)
MailMessage.From = New MailAddress("abcd@xxx.com")
MailMessage.Subject = mailsubject
MailMessage.Body = mailbody

Dim SmtpClient As New SmtpClient()
SmtpClient.Send(MailMessage)

in my web.config

<system.net>
<mailSettings>
<smtp from="abcd@xxx.com">
<network host="relay-hosting.secureserver.net" port="25" userName="myusername" password="mypassword" />
</smtp>
</mailSettings>
</system.net>