I believe this redirect will work for you to only redirect from http to https (but keeping the rest of the url unchanged:
RewriteEngine On
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^/(.*) https://%{SERVER_NAME}/$1 [L,R]
You'll need another rewrite for the www -> mail instance.
There are other options at
http://www.askapache.com/htaccess/ap...-examples.html