Explicitly Mapping MAPI Calls to MAPI DLLs

In some cases, MAPI calls made from a particular DLL or executable file need to be routed to the system MAPI DLL (called Mapi32x.dll) or another custom MAPI DLL even though the default mail client supports the call. Such DLLs or executables files can be listed as string registry values in the HKLM\Software\Microsoft\Windows Messaging Subsystem\MSMapiApps key. The registry value for these keys can be empty or identify a mail client key that resides under HKLM\Software\Clients\Mail.

When the stub library resolves a MAPI call, it first enumerates the DLL and executable files listed under the HKLM\Software\Microsoft\Windows Messaging Subsystem\MSMapiApps key checking to see if the DLL or executable file is currently in process. If there is a match, the stub library gets the string value. If the value is the empty string, the stub library routes the call to the system MAPI DLL, Mapi32x.dll. If the string is not empty, the stub uses the string value to find the key below HKLM\Software\Clients\Mail where it can find the appropriate registry value to dispatch the call, one of DLLPath, DLLPathEx, or MSIComponentID values. For example:

HKLM\Software\Microsoft\Windows Messaging Subsystem\MSMapiApps::exchng32.exe = ""  
  (route call directly to Mapi32x.dll)

HKLM\Software\Microsoft\Windows Messaging Subsystem\MSMapiApps::msspc32.dll = "Microsoft Outlook"
   (route call using Microsoft Outlook key under HKLM\Software\Clients\Mail)