Решение:from email.mime.text import MIMEText
from email.mime.multipart import MIMEMultipart
from email.charset import Charset
from email.encoders import encode_quopri
SERVER = 'mailserver.loc'
FROM = 'billing@' + SERVER
TO = 'abonent1@' + SERVER
msg = MIMEMultipart('mixed')
msg['Subject'] = 'subject'
msg['From'] = FROM
msg['To'] = TO
msg['X-Message-id'] = 'xmsg_id'
charset = Charset('windows-1251')
att1 = MIMEText('проверка111 проверка проверка', 'plain', _charset=None)
encode_quopri(att1)
att1.set_charset(charset)
del att1['Content-Transfer-Encoding']
del att1['MIME-Version']
att1['Content-Transfer-Encoding'] = 'quoted-printable'
msg.attach(att1)
print (msg.as_string())
################### OUTPUT ###################
# Content-Type: multipart/mixed; boundary="===============2009055196=="
# MIME-Version: 1.0
# Subject: subject
# From: billing@mailserver.loc
# To: abonent1@mailserver.loc
# X-Message-id: xmsg_id
# --===============2009055196==
# Content-Type: text/plain; charset="windows-1251"
# Content-Transfer-Encoding: quoted-printable
# =D0=BF=D1=80=D0=BE=D0=B2=D0=B5=D1=80=D0=BA=D0=B0111=20=D0=BF=D1=80=D0=BE=D0=
# =B2=D0=B5=D1=80=D0=BA=D0=B0=20=D0=BF=D1=80=D0=BE=D0=B2=D0=B5=D1=80=D0=BA=D0=
# =B0
# --===============2009055196==--