Can an IFilter be written to search Thunderbird or other email apps? I thought it only worked on files.
Assuming MSN Desktop uses the same architecture as the rest of the MSSearch family, you have IFilters to handle specific file types, and Protocol Handlers that go and find them - a protocol handler to handle Thunderbird could probably be written.
For some reason, IFilters can only be written in C++ right now, not .NET. Annoying.