It is not documented anywhere of course. That's why there is MAPI-L list and
this newsgroup :-)
It worked in all versions of Exchange (5.5, 2000, 2003, I did not test 5.0)
as well as in the SMTP provider in all versions of Outlook in my tests.
Dmitry Streblechenko (MVP)
OutlookSpy - Outlook, CDO
and MAPI Developer Tool