The MAPICODE.H header file contains many of the values that a client or service provider might return from an interface method implementation or might see returned from a call. These values are SCODE values rather than HRESULT values. The OLE SCODE value S_OK is used to represent success in MAPI calls.
The codes to represent warning and failure conditions follow a different naming convention that begins with the prefix MAPI, an underscore, and either a W or an E to indicate the type of code. The rest of the code is a short character string to describe the condition. Each word in the string is separated by an underscore. For example, the error value MAPI_E_TOO_COMPLEX indicates that the implementation could not handle whatever was being requested in the call. The warning value MAPI_W_PARTIAL_COMPLETION indicates that the call succeeded, but that there were problems. Only part of the operation was completed successfully.