I never used the email package, so my issue is maybe not a bug. I'm 
trying to send an email with diacritics in the subject and the body. 
I'm french so it's natural to use characters not in the ASCII range. I 
wrote this small program:

def main():
    # coding: utf8
    ADDRESS = 'victor.stinner@haypocalc.com'
    from email.mime.text import MIMEText
    msg = MIMEText('accent éôŁ', 'plain', 'utf-8')
    msg['Subject'] = 'sujet éôł'
    msg['From'] = ADDRESS
    msg['To'] = ADDRESS
    text = msg.as_string()
    print("--- FLATTEN ---")
    print(text)
    return
    import smtplib
    client=smtplib.SMTP('smtp.free.fr')
    client.sendmail(ADDRESS, ADDRESS, text)
    client.quit()
main()

(remove the "return" to really send the email)

The problem:
  (...)
  File "/home/haypo/prog/py3k/Lib/email/generator.py", line 141, in 
_write_headers
    header_name=h, continuation_ws='\t')
  File "/home/haypo/prog/py3k/Lib/email/header.py", line 189, in 
__init__
    self.append(s, charset, errors)
  File "/home/haypo/prog/py3k/Lib/email/header.py", line 262, in 
append
    input_bytes = s.encode(input_charset, errors)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 
6-8: ordinal not in range(128)

I don't understand why it uses ASCII whereas I specified that I would 
like to use the UTF-8 charset.

My attached patch reused the message charset to encode the headers, 
but use ASCII if the header can be encoded as ASCII. The patch 
included an unit test.