See: microsoft.public.win32.programmer.messaging
That is a newsgroup for programmers. That might help.
Also see:
http://msdn.microsoft.com/en-us/library/ms709546(VS.85).aspx
Note that the documentation says "Do not use" in many places. It is probably
safe to use them except assume none of it will work with future versions of
Windows, since it is nearly guaranteed that the API will not work in future
Windows.
I am not sure what you need to do so I am not sure if any of the above is
useful to you. If Acu Cobol must have an IDispatch interface then it is
probable that Acu Cobol cannot use the OE API directly. There is an IDL file
for the OE API in the Windows SDK; if you can figure out how to generate a
type library from an IDL and if Acu Cobol can access a COM object using an
IUnkown interface, then it might be possible to do what you need to do.
There is something else called Simple MAPI; I think (I am not sure) it can
be used with Windows Mail. See the following for other possibilities.
http://msdn.microsoft.com/en-us/library/aa139844.aspx