from email.policy import SMTP, SMTPUTF8
from email.message import Message

m = Message()
m.add_header('Subject', 'üöä')

print(m.as_string())

print(SMTP.utf8)
print(m.as_string(policy=SMTP))

print(SMTPUTF8.utf8)
print(m.as_string(policy=SMTPUTF8))