Mendes,
bom dia.
Dependerá da versão:
HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Outlook
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows Live Mail
HKEY_CURRENT_USER\Software\Microsoft\Exchange
HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\Windows Messaging Subsystem\Profiles\Outlook