I’ve mentioned this before, in my About Me page, but I just tried out a more current release of OpenChange and the Evolution mapi plugin, from this source: http://download.opensuse.org/repositories/home:/msuman:/evolution-mapi-provider/
This thing is so close to being totally sorted. I’m really impressed. The current build from the OpenSuse build farm service (19th Sept) has problems displaying the message body, but they are aware of this problem and believe it was introduced in a recent patch, so should be resolved shortly.
This is great stuff.
I had a chat with one of the developers on the #openchange IRC channel on Freenode, and he told me that RPC/HTTP support may well be here in the future!
The downloads (pre-built packages) from the OpenSuse build farm service are temporarily unavailable. I believe this is just until the re-licensing of Evolution is finished. In the meantime you could download source and compile for yourself.