GMail + C# + Web.Config: Send Mail Works Programmatically, Throws Exception Using Web.Config Values

Given the following section in Web.Config:

<system.net>
    <mailSettings>
        <smtp deliveryMethod="Network" from="SomeWebsite Admin &lt;someuser@gmail.com&gt;">
            <network host="smtp.gmail.com" port="587" defaultCredentials="true" userName="someuser@gmail.com" password="somepassword" />
        </smtp>
    </mailSettings>
</system.net>

And the following code snippet:

                smtp   = new SmtpClient();

                smtp.Host = "smtp.gmail.com";
                smtp.Port = 587;
                smtp.EnableSsl = true;
                smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
                smtp.UseDefaultCredentials = false;
                smtp.Credentials = new NetworkCredential( "someuser@gmail.com", "somepassword" );

                smtp.Send( mailMessage );

The above works fine, however commenting out the programmatic overrides, like this:

                smtp   = new SmtpClient();

                //smtp.Host = "smtp.gmail.com";
                //smtp.Port = 587;
                smtp.EnableSsl = true;
                //smtp.DeliveryMethod = SmtpDeliveryMethod.Network;
                //smtp.UseDefaultCredentials = false;
                //smtp.Credentials = new NetworkCredential( "someuser@gmail.com", "somepassword" );

                smtp.Send( mailMessage );

fails with:

System.Net.Mail.SmtpException: {"The SMTP server requires a secure connection or the 
client was not authenticated. The server response was: 5.5.1 Authentication Required. 
Learn more at                              "}

Yes, the blank space after the "learn more at" is really there, and it makes things extra fun. Thing is, I can't be hardcoding these values, as they change from development to internal staging to live. So I'm wondering why it appears to fail to