When you first run the Mail application, it asks whether you want to use an existing postoffice on the workgroup or to create the postoffice on your computer. If you accidentally select the incorrect option, you will need to reinitialize Mail so that these options are available again. You can also use this procedure to recreate a postoffice.
To reinitialize Mail, follow these steps:
HKEY_CURRENT_USER\Software\Microsoft\Mail\Microsoft Mail
CustomInitHandler:REG_SZ:WGPOMG32.DLL, 10