I am aware of the feature to open two libraries and drag items form one the other. However, I am dealing with tens of thousand of emails, and the re-indexing seems to take just about as long as the initial importing into a library.
Now wishing I had separated documents and emails from the get go, I wonder if it would be feasible to copy the folder structure of the source library to a new folder structure for the target library, then to just delete the respective elements that I don’t want.