I've been working with MAPI for over 5 years now. In that 5 years I've supported my share of developers writing their own MAPI providers (message store, transport, address book), but I've never actually sat down and written one myself. Until now ;-)
I decided to do a transport, since we already *kinda* have a message store, and we don't really get a lot of support cases for address book providers.
So, here it is. I based the actual delivery mechanism on the PeerXP sample from way back. Messages are stored as files on disk, and e-mail addresses are UNC share paths.
Development was done with Visual Studio 2003 (unmanaged C++ of course!) and testing was done with Outlook 2003 SP2. I included my development notes with the source, and there's a README on basic installation and usage.
Feedback is welcome!