>>> from email.mime.text import MIMEText
>>> from email.header import Header
>>> msg = MIMEText('dummy')
>>> h = Header('dummy\nX-Injected-Header: yes')
>>> msg['Subject'] = h
>>> print msg.as_string()
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: dummy
X-Injected-Header: yes

dummy