sending mail using tls using Emacs under Windows

[Will Parsons]

I'd like to be able to send mail using Emacs 24.2.1 under Windows and
am having difficulties.  With the following in my .emacs file:

(setq
 send-mail-function 'smtpmail-send-it
 smtpmail-smtp-server "smtp.gmail.com"
 smtpmail-smtp-service 587)

and a suitable entry in .authinfo:

 machine smtp.gmail.com login xxxx@gmail.com port 587 password xxxxx

attempting to send a message using the default method "Gnus Message"
results in:

 smtpmail-send-it: Sending failed: 530 5.7.0 Must issue a STARTTLS
 command first. y142sm1414876iod.25 - gsmtp

Looking under Options => Packages, I see that gnutls is built-in, but
running gnutls-available-p returns nil.

Am I missing something?

-- 
Will