Comment 1 for bug 1445667

seems to be a specific problem with 10.10.3 and Mail.app, since App.open() generally works.

I tried other ways, to start Mail from SikuliX or from terminal: all not working.

the only, that worked in the Terminal was:
osascript -e "tell application \"Mail\" to activate"

(cannot be issued currently with the run() feature - working on that)

once Mail is running, App.close() and App.focus() still work.