These instructions provided by Ray Fong:
1. Open LCS, right click User Services Global Settings, Properties. Under the Domains tab, click Add, and add the .com domain (i.e. domain.com)
2. Under LCS again, in the Users folder, verify the user is enabled, and the SIP URI is set to sip:email@example.com
3. Go to DNS, create a forward lookup zone called “domain.com”.
4. Right-click domain.com, Other New Records, select Service Location (SRV). For the Service, type _sip, for the Port number, type 5060, for the host offering this service, type the FQDN of the server. (Like the attached picture)
5. Ipconfig/flushdns, ipconfig/registerdns.
6. Go to LCS, right click the server, select Stop, then Start.
7. Logon to the IM (Windows Messenger 5.0) using the username firstname.lastname@example.org under SIP Communications Service Account.
As far as SharePoint, you need to go to http://companyweb, Site Settings,
Go to Site Administration, Manage Site Collection Users. Update the email address to @domain.com. Also, the client needs Office 2003 installed. If you don’t have Office 2003, Outlook 2003 is fine also.
Microsoft SBS Product Support