Outlook has decided to publish documentation for a number of previously internal only APIs which ship with Outlook 2003 SP1. Here's a link to the docs.
Due to a tight deadline, we were only able to document a small subset of these APIs. We are considering plans in place to expand these docs over time, but nothing is fixed in stone.
Note that some of the APIs being documented may be present in other versions of Outlook, but we're only providing support for their use in Outlook 2003 SP1. While it is likely that we will decide to support these APIs in future versions of Outlook, we will NOT provide support for earlier versions (including any version of Outlook 2000 and Outlook XP and Outlook 2003 RTM). There are both technical and resource related reasons for this. Some highlights of the new docs
Hit me with any questions you have about these APIs.
<2:25pm - fixed link>
[Update: This was superseded by the Outlook 2007 Auxiliary Reference]
(Note: I accidently deleted a comment asking if the MAPI<->MIME API can be used in a service. Sorry about that.)
I'm not aware of any problems with the MAPI<->MIME API in relation to use in a service or in multithreaded applications.