Extension Manager documents (other than API docs)?

I’m currently writing an extension manager plugin for mail virus scanning. I’m using EM_NSFNOTEUPDATEMAILBOX, but that isn’t working the way I expect it to (see my previous post), so I’m looking around for alternatives.

I notice that the various EM_* hooks aren’t particularly well documented, particularly the data that is available via EMRECORD->Ap - the docs and headers are silent on what most callbacks get.

Is there any better information around?

Thanks,

Rob.