See: Remote Web Workplace Part 1 (PDF) for more info
Configure Internet and Email access settings to allow access to the Remote Web Workplace web service from the Internet (Server Management -> Connect to the Internet -> Web Services Configuration)
Configure the IIS Web application named Remote to Allow access by Default ( ISS Manager -> Web Sites -> Default Web Site -> Remote -> Properties -> Directory Security -> IP address and Domain Name restrictions -> Edit)
Configure the Firewall to allow SSH (Port 443) to the SBS box from the WAN. Create a custom service for RWW (Port 4125) and allow access to that service from the WAN to the SBS box.
Connecting client machines must be Windows XP Pro or better
Access the site to test:
From the LAN https://SBSSERVER/Remote
From the WAN: https://mail.domainname.com/Remote