Lotus Notes integration with .Net

It might take time to learn how to use IBM documentation but I am confident you will find the documentation you need of everything. Unless you need documentation that they keep secret. They used to call those "Program Logic Manuals"; at least they did back in the early 1970s.

Something to look for are Red Books or Redbooks; they usualy have information that cannot be found elsewhere.

There is a huge customer base for Lotus Notes; you should be able to get plenty of help from cutomer groups and forums and such. The groups were quite extensive even before there was much of an internet. IBM's PROFS was one of the earliest email systems. I used in in the late 1980s.