SMTP Client - 'The SMTP host was not specified"

I am trying to send an email using the SMTPClient class but I keep getting the 'SMTP host was not specified error'. This is the code:
                    Dim smtpServer As SmtpClient
                    smtpServer = New SmtpClient()
                    smtpServer.Send(emailMessage)
                    smtpServer = Nothing
where emailMessage is an object of type MailMessage and is populated earlier in the function.
 
When I instantiate the object like this:
     smtpServer = New SmtpClient("our.host")
it works correctly.

As I understand it, the SMTPClient should obtain the host from the config file:
<configuration>
  <system.net>
    <mailSettings>
      <smtp deliveryMethod="network">
        <network host="our.host"/>
      </smtp>
    </mailSettings>
  </system.net>
</configuration>

The config file also contains system.diagnostic and runtime settings which I've left out for clarity.

Incidentally the framework is 2.0.

Any help would be much appreciated