In Exchange System Manager, go to Servers -> Server Name -> Protocols -> SMTP -> Default SMTP Virtual Server
Right click on the Default SMTP virtual server and choose properties.
Click on advanced and click add. Enter 587 in the TCP port and hit OK. It'll look like this when done:
Click on the access tab. Click on Certificate.
Put your existing 3rd party SSL certificate in place on this virtual server. This must be a 3rd part SSL certificate (like godaddy).
And that's it.
When configuring the email client, tell it to use authentication and SSL over port 587. In Outlook 2007, use AUTO for the type of encryption.