Mac OS X smtpmail "encryption too weak"

[Gilbert Harman]

I used to be able to use emacs to connect to the mail server at my
university, but recently the university smtp server has required
authentication and now I get the following error: "encryption too weak"

503 5.7.0 encryption too weak 0 less than 39
221 2.0.0 smtpserver2.Princeton.EDU closing connection
smtpmail-send-it: Sending failed; SMTP protocol error

I am using Mac OS 10.2.1 and emacs 21.3.50.1.  In addition to the rmail
stuff that used to work, my .emacs file includes:

(setq smtpmail-auth-credentials
      '(("smptauth.princeton.edu" 25 "myusername" "mypassword")))
(setq smtpmail-starttls-credentials
      '(("smptauth.princeton.edu" 25 "~/.my_smtp_tls.key"
         "~/.my_smtp_tls.cert")))

I have no trouble using the Mac Mail program to send and receive mail using
the new authentication.  But I can't seem to get mail working from within
emacs.

 gil