from email.message import EmailMessage
from email.contentmanager import raw_data_manager
m = EmailMessage()
content = "\u65e5\u672c\u8a9e\n"
raw_data_manager.set_content(m, content, charset='shift_jis')
print(m)

Traceback (most recent call last):
  File "<python-input-0>", line 7, in <module>
    print(m)
    ~~~~~^^^
  File "/home/serhiy/py/cpython/Lib/email/message.py", line 1015, in __str__
    return self.as_string(policy=self.policy.clone(utf8=True))
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/email/message.py", line 1012, in as_string
    return super().as_string(unixfrom, maxheaderlen, policy)
           ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/email/message.py", line 194, in as_string
    g.flatten(self, unixfrom=unixfrom)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/email/generator.py", line 118, in flatten
    self._write(msg)
    ~~~~~~~~~~~^^^^^
  File "/home/serhiy/py/cpython/Lib/email/generator.py", line 183, in _write
    self._dispatch(msg)
    ~~~~~~~~~~~~~~^^^^^
  File "/home/serhiy/py/cpython/Lib/email/generator.py", line 220, in _dispatch
    meth(msg)
    ~~~~^^^^^
  File "/home/serhiy/py/cpython/Lib/email/generator.py", line 258, in _handle_text
    msg.set_payload(msg._payload, charset)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/email/message.py", line 361, in set_payload
    self.set_charset(charset)
    ~~~~~~~~~~~~~~~~^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/email/message.py", line 392, in set_charset
    self._payload = charset.body_encode(self._payload)
                    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/home/serhiy/py/cpython/Lib/email/charset.py", line 441, in body_encode
    string = string.encode(self.output_charset).decode('ascii')
             ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'iso2022_jp' codec can't encode character '\udc93' in position 0: illegal multibyte sequence
encoding with 'iso-2022-jp' codec failed