Open the OCS admin tool. Right click the forest and select Global Properties. The first tab that appears will show you the configured SIP domains in your environment. By default only your forest's domain name is there. Add your SMTP domain name and any others that you need. Once that change is saved and replicated you will be able to use that domain for user's SIP addresses.
Mike Stacy | Evangelyze Communications | http://www.evangelyze.net/cs/blogs/mike