Parallels Pro Control Panel supports mail add-ons that are based on the Mail API commands included in Ensim WEBppliance Pro 3.X for Windows 2003. This backward compatibility feature can be enabled using the 3.XCompliantAPI parameter while registering the mail add-on with the RegisterMailAddon command.
In backward compatibility mode, the existing mail add-ons are supposed to work as is with Parallels Pro Control Panel. Exceptions to this include: