SmtpClient Won't Authenticate When Inflated From Web.config

When using the system.net/mail web.config settings to configure my SmtpClient, it fails to deliver emails, with an "protocol error" described best by Base64 encoding and authentication problems:

Example:
With the following Config
<system.net>
<mailSettings>
<smtp from="[URL]">
<network host="[URL]"
port="2525"
defaultCredentials="false"
userName="username"
password="password"/>
</smtp>
</mailSettings>
</system.net>

And the Code:

var tmp = new SmtpClient();
MailMessage msg = new MailMessage();
msg.Subject = "test";
msg.From = new MailAddress("[URL]");
msg.To.Add(new MailAddress("[URL]"));
msg.Body = "test";
tmp.Send(msg);

Produces the error message:

System.Net.Mail.SmtpException: The server committed a protocol violation The server response was: UGFzc3dvcmQ6
at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, SmtpFailedRecipientException
& exception)
at System.Net.Mail.SmtpClient.Send(MailMessage message)

However, in the following code where I manually set all the properties, the code runs without exception and the email is delivered.

var tmp2 = new SmtpClient("[URL]", 2525);
tmp2.Credentials = new NetworkCredential("username", "password");
tmp2.UseDefaultCredentials = false;
MailMessage msg = new MailMessage();
msg.Subject = "test";
msg.From = new MailAddress("[URL]");
msg.To.Add(new MailAddress("[URL]"));
msg.Body = "test";
tmp2.Send(msg);