Choose a specific version of MAPI to load

Applies to: Outlook 2013 | Outlook 2016

When linking explicitly to an implementation of MAPI, you must carefully select which implementation to load.

There are two methods to link explicitly to an implementation of MAPI.

  1. Load the MAPI stub library, and specify in the registry a custom DLL to load and dispatch MAPI calls to. Or,

  2. Implement the MAPI client lookup algorithm to look up the version of MAPI used by the default mail client and load it.

Because you can change the Mapi32.dll Stub Registry Settings to direct your application to use any implementation of MAPI, we recommend that you direct your application to use an implementation of MAPI that you have tested with. The following describes both methods of linking explicitly.

Read MAPI implementation information from the registry

  1. The registry keys that indicate a custom DLL for a mail client are located under the HKLM\Software\Clients\Mail key of the mail client.

    The following table describes these keys:

    Key Description
    A Windows Installer PublishComponent category ID (GUID) that identifies the DLL that exports simple MAPI or MAPI calls. If set, this key takes precedence over the DLLPath or DLLPathEx key.
    Locale identifier (LCID) for your application. The first string value identifies a sub-key from HKLM\Software and subsequent string values identify registry values underneath this key that contain locale information.
    LCIDs for Microsoft Office. The first string value identifies a sub-key from HKLM\Software and subsequent string values identify registry values underneath this key.

    Obtain the information from these keys.

  2. Pass the values that you obtained from the previous step to the FGetComponentPath function. FGetComponentPath is a function that is exported by the MAPI stub library Mapistub.dll. It returns the path of the custom version of MAPI.

Loading the implementation of MAPI marked as default

  1. Read the HKLM\Software\Clients\Mail::(default) registry value.

  2. Look up the information for the indicated client as described earlier.


Note that the default mail client might not implement Extended MAPI.


To load MAPI as implemented by Outlook, look up the registry keys under HKLM\Software\Clients\Mail\Microsoft Outlook and pass them to FGetComponentPath. FGetComponentPath will return the path for Outlook's implementation of MAPI.

If the keys MSIComponentID, MSIApplicationLCID, and MSIOfficeLCID are not set, check the DLLPathEx registry value. If the keys are set, FGetComponentPath gives the path of the client's implementation of MAPI.

Implementing the MAPI Client Lookup algorithm

The following table lists the four functions from MFCMAPI that are used to look up the path for a custom implementation of MAPI:

Function Description
Gets the MAPI library path.
Gets the MAPI mail registry key.
Gets the Windows Installer identifier.
Gets the component path using FGetComponentPath.

Because MFCMAPI loads the default implementation of MAPI by default, if you want to use a different implementation of MAPI, you must explicitly direct it to do so. This is performed by using the Session\Load MAPI routine.

How these functions work

  1. MFCMAPI calls GetMAPIPath, passing NULL for the client parameter, to load the default MAPI implementation.

  2. GetMAPIPath calls GetMapiMsiIds to read the values for MSIComponentID, MSIApplicationLCID, and MSIOfficeLCID.

  3. GetMapiMsiIds calls GetMailKey to open the registry key for the default mail client.

  4. GetMapiMsiIds uses the registry handle returned by GetMailKey to look up values for MSIComponentID, MSIApplicationLCID, and MSIOfficeLCID.

  5. The values for MSIComponentID, MSIApplicationLCID, and MSIOfficeLCID, are returned to GetMAPIPath. GetMAPIPath then passes them to GetComponentPath.

  6. GetComponentPath loads the MAPI stub library, Mapi32.dll, from the system directory.

  7. GetComponentPath then retrieves the address of the FGetComponentPath function from Mapi32.dll, assuming that Mapi32.dll exports FGetComponentPath.

  8. If getting the address of FGetComponentPath from Mapi32.dll fails, GetComponentPath retrieves the address from Mapistub.dll.

  9. GetComponentPath then calls FGetComponentPath, obtaining the path of the default version of MAPI.

  10. GetMAPIPath then returns this path to the caller, which then loads MAPI and explicitly links to it as described in Link to MAPI Functions.


  • To support localized copies of MAPI for English and non-English locales, GetMAPIPath reads the values for the MSIApplicationLCID and MSIOfficeLCID subkeys. GetMAPIPath then calls FGetComponentPath, first specifying MSIApplicationLCID as szQualifier, and again specifying MSIOfficeLCID as szQualifier. For more information about registry keys for mail clients that support non-English languages, see Setting Up the MSI Keys for Your MAPI DLL.
  • If MFCMAPI does not receive a path for MAPI using GetMAPIPath, it loads the MAPI stub library from the system directory.
  • The MSMapiApps registry value discussed in Explicitly Mapping MAPI Calls to MAPI DLLs only applies when the MAPI Stub library is used. Applications that load a specific implementation of MAPI or load the default implementation do not have to set the MSMapiApps registry key.

See also