forcing http to go to https

I'm using IMP for web email and I want to force all connections to go via SSL.

Here's the relevant parts of httpd.conf:

Alias /horde/ /var/www/html/horde/

<VirtualHost *:443>
   ServerAdmin  webmaster@xxxxxxxxxxxx
   DocumentRoot /var/www/html/horde/
   ServerName webmail.mydomain.com
   Options Indexes FollowSymLinks
</VirtualHost>

#<Directory /var/www/html/horde/>
#    RewriteEngine       on
#    RewriteCond         %{SERVER_PORT} !=443
#    RewriteRule         ^.* - [F]
#    ErrorDocument       403 https://webmail.mydomain.com
#</Directory>

If you connect to https://webmail.mydomain.com, it works just fine.  I want to
make sure than any connection to http://mydomain.com/horde gets forced to use
the https://webmail connection.

However, if I enable the rewrite for the horde directory, any attempt to
access that directory results in an error, "Maximum redirection exceeded".
I'm sure there's a problem in my thinking. Please lead me to the path of
enlightenment.
--
   Steve