import email.message
from email import message_from_bytes
from email.policy import default

reference_ids = [
    '<BN8PR17MB27372595A957D7912CEE184FBF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    '<CAM9Ku=FZ5RGMvw3VzNrZz+DA78zyq2Am8fz9JNLmjUQ9ZEXpDQ@mail.gmail.com>',
    '<BN8PR17MB27371C71A65834531DF028BBBF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # As a test, copied I successively shortened this one
    # and sure enough, once it gets short enough, it no longer gets encoded
    # '<BNPR17MB27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BNR17MB27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BN17MB27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BN7MB27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BNMB27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BNB27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BN27377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BN7377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    # '<BN377F609B669D0E72638D69BF6F9@BN8PR17MB2737.namprd17.prod.outlook.com>',
    '<CAM9Ku=E1wmpj=AMRhsh-Sk1RHqmK_x-J5ey8szVehefYQvn13w@mail.gmail.com>']
in_reply_to = reference_ids[0]

class Request:
    """fugly hack to let the code below work as in the original question"""
    def __init__(self):
        self.vars = self
        self.email = "sender@example.org"
        self.to = "recipient@example.net"
        self.cc = "cc.recipient@example.com"
        self.bcc = "bcc.recipient@example.int"
        self.subject = "I have seen the fnörds"
        self.message = """\
LANGUAGE (NATIVE NAME)	HELLO
----------------------	-----
Japanese (日本語)	こんにちは / ｺﾝﾆﾁﾊ
Chinese (中文,普通话,汉语)	你好
Cantonese (粵語,廣東話)	早晨, 你好
Korean (한글)	안녕하세요 / 안녕하십니까
"""

request = Request()
es = request

msg = email.message.EmailMessage()
if reference_ids is not None:  # no parentheses necessary, this is not C
    msg.add_header('In-Reply-To', in_reply_to)
    msg.add_header('References', (' ').join(reference_ids))
msg['Subject'] = request.vars.subject
msg['From'] = es.email
msg['To'] = request.vars.to
msg['CC'] = request.vars.cc
msg['BCC'] = request.vars.bcc
msg.set_content(request.vars.message)

print('** Original')
print(msg.as_string())

print("-" * 72)

copy = message_from_bytes(msg.as_string().encode('us-ascii'))
print('** Original, from message_from_bytes')
print(copy.as_string())

print("-" * 72)

custom_policy = default.clone(max_line_length=100)

msg = email.message.EmailMessage(policy=custom_policy)
if (reference_ids is not None):
    msg.add_header('In-Reply-To', in_reply_to)
    msg.add_header('References', (' ').join(reference_ids))
msg['Subject'] = request.vars.subject
msg['From'] = es.email
msg['To'] = request.vars.to
msg['CC'] = request.vars.cc
msg['BCC'] = request.vars.bcc
msg.set_content(request.vars.message)

print('** Fixed, with custom policy')
print(msg.as_string())

print("-" * 72)

copy = message_from_bytes(msg.as_string().encode('us-ascii'), policy=custom_policy)
print('** Fixed, with custom policy; from message_from_bytes')
print(copy.as_string())