authentication when sending mail using emacs

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Hi,

I am trying to send email from emacs (GNU Emacs 21.3.1)
using Earthlink as my ISP.  I have the following relevant
settings in my .emacs file:

(setq send-mail-function 'smtpmail-send-it)
(setq smtpmail-default-smtp-server "smtpauth.earthlink.net")
(setq smtpmail-auth-credentials
      '(("smtpauth.earthlink.net" 25 "<username>@earthlink.net"
                                  "<password>")))
(setq smtpmail-starttls-credentials
      '(("smtpauth.earthlink.net" 25 nil nil)))

However, when I go to send email, I always get the following
error message:

550 Please configure your mail client to use authentication.

Any ideas what might be causing this and how to fix it?

TIA!!!