Since not all of my clients use authenticated SMTP accounts, I resorted to using the SMTP account only if app key values are supplied in web.config file.
Here is the VB code:
sSMTPUser = ConfigurationManager.AppSettings("SMTPUser")
sSMTPPassword = ConfigurationManager.AppSettings("SMTPPassword")
If sSMTPUser.Trim.Length > 0 AndAlso sSMTPPassword.Trim.Length > 0 Then
NetClient.Credentials = New System.Net.NetworkCredential(sSMTPUser, sSMTPPassword)
sUsingCredentialMesg = "(Using Authenticated Account) " 'used for logging purposes
End If
NetClient.Send(Message)