UnicodeDecodeError: 'utf8' codec can't decode byte 0xf6 in position 380: invalid start byte

just tried sending a mail with the german umlaut "ö" included in the body:

sending went fine but i couldn't save it to the "sent" folder

log + mail attached

(from redmine: created on 2013-09-26, closed on 2013-10-02)