Ingo I am assigning this directly to you because I also want to hear your opinion with a thought to accessibility.
I am not a fan of the current notepad recipients. I would rather have a split widget with a vertical split between notepad and recipients with a moveable splitter. So that users can decide to enlarge the notepad or the recipients by moving the splitter.
Originally I had chosen the Tab Widget because I planned to have the Notepad as MIME Reader which would also show attachments and other parts of the MIME Structure. That is still a goal but I think in this case movable splitters are better then tabs.