Presence info for an OCS user may not appear, or may appear intermittently, in Outlook when the SIP address and primary e-mail addresses differ

Thankfully, there is a relatively simple workaround for Outlook 2007 and an additional registry setting needed for Outlook 2003 SP2.

Step 1 (Exchange 2000/2003 only): Add a SIP proxy address in Exchange Server for the user account that specifies the user's SIP URI.

Step 2 (Outlook 2003 SP2 only):

By default, Outlook 2003 SP2 does not search for the user's SIP address to get the presence information.

1. Click Start, click Run, type regedit, and then press ENTER.
2. Locate the following registry subkey: HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Common\PersonaMenu
3. Click Edit, point to New, and then click DWORD value.
4. Type QueryServiceForStatus, and then click OK.
5. Double-click QueryServiceForStatus, type 2, and then click OK.
6. Click Edit, point to New, and then click DWORD value.
7. Type EnableDynamicPresence, and then click OK.
8. Double-click EnableDynamicPresence, type 1, and then click OK.
9. Exit Registry Editor.

After you create these registry keys, Outlook 2003 SP2 shows presence information.

You can read more about it here.