I used the following rewrite rules to redirect squirrelmail from http to https automatically. I added this to the end of the /etc/httpd/conf.d/squirrelmail.conf in RHEL5.x.

vi /etc/httpd/conf.d/squirrelmail.conf

RewriteEngine On
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^/(.*) https://%{SERVER_NAME}/$1 [L,R]