Exchange 2003 SMTP TLS connection

Here is the scenerio.  We recently implemented a third party spam filter, our MX record points to them they clean the mail and forward it to us.  We have still been getting a ton of spam that is coming directly to our mail server.  We cant lock down port SMTP because I have mobile users who connect via pop and SMTP for their mail.  I created a second SMTP virtual server on port 2525 and have all mail from our spam company sent to through that connector and I am trying to set the original virtual server on port 25 to only accept encrypted connections but am not having any success.  We have an existing root certificate (owa.redwoodit.com) that we use for RPC and Outlook Web Access.  I installed that certificate to the SMTP virtual connector checked the Require Secure Channel check box and set the Authentication to Basic and Require TLS encryption.  I setup an Outlook 2003 client to connect but I am getting the following error "establishing an encrypted connection to your outgoing (SMTP) server failed".  Any help or suggestions would be greatly appreciated as I am stumped on where to go from here.