В чем может быть ошибка?

try:
                            mess = str(msg).decode("utf-8")
                        except UnicodeEncodeError:
                            mess = str(msg)
                        except UnicodeDecodeError:
                            mess = str(msg)
                        try:
                            with open(os.path.join(DIR,".".join([name,"eml"])),"a") as f:
                                f.write(mess) 
                        except IOError:
                            name = "@noSubject@ " + str(random.getrandbits(100))
                            with open(os.path.join(DIR,".".join([name,"eml"])),"a") as f:
                                f.write(mess)

UnicodeEncodeError: 'ascii' codec can't encode...

f.write(mess)