Can't do email in Emacs 24.3

[Joe Fineman <joe_f@verizon.net>]

I've been using Emacs 22.3 (on Windows XP Professional SP3 with Cygwin)
with an SMTP email connection to a Verizon DSL account successfully
since 2008.  The outgoing SMTP server is outgoing.verizon.net, and the
port number is 25.  In my .emacs file, I have been using the
smtpmail-auth-credentials variable to provide emacs with the SMTP server
name, port number, user name and password of the email account.  This is
a plain (unencrypted) connection, not using SSL/TLS.

The other day, I attempted to ugrade to Emacs 24.3, and all went well
until I tried to send email.  Reading the documentation, I have
discovered that the variable "smtpmail-auth-credentials" is no longer
used, and I have created a ~/.authinfo file containing the required
authentication information.  I have also added variable definition (setq
smtpmail-stream-type nil) though this should presumably not be necessary
for a plain connection.

In all cases (and this is also true for Emacs 24.1, which I have also
tried), any attempt to send an email gives the response

  Sending failed: 500 5.7.0 Unknown AUTH error -1
  (Internal authentication error) in response to
  <a very long string of characters>

Can anyone explain how to fix this?  Thanks very much.
-- 
---  Joe Fineman    joe_f@verizon.net

||:  Idealism, frustration, demoralization.  :||