As of Python 3.2, released Nov 2011, the smtplib has a new function send_message instead of just sendmail, which makes dealing with To/CC/BCC easier. Pulling from the Python official email examples, with some slight modifications, we get:
# Import smtplib for the actual sending function
import smtplib
# Import the email modules we'll need
from email.message import EmailMessage
# Open the plain text file whose name is in textfile for reading.
with open(textfile) as fp:
# Create a text/plain message
msg = EmailMessage()
msg.set_content(fp.read())
# me == the sender's email address
# you == the recipient's email address
# them == the cc's email address
# they == the bcc's email address
msg['Subject'] = 'The contents of %s' % textfile
msg['From'] = me
msg['To'] = you
msg['Cc'] = them
msg['Bcc'] = they
# Send the message via our own SMTP server.
s = smtplib.SMTP('localhost')
s.send_message(msg)
s.quit()
Using the headers work fine, because send_message respects BCC as outlined in the documentation:
send_message does not transmit any Bcc or Resent-Bcc headers that may appear in msg
With sendmail it was common to add the CC headers to the message, doing something such as:
msg['Bcc'] = [email protected]
Or
msg = "From: [email protected]" +
"To: [email protected]" +
"BCC: [email protected]" +
"Subject: You've got mail!" +
"This is the message body"
The problem is, the sendmail function treats all those headers the same, meaning they'll get sent (visibly) to all To: and BCC: users, defeating the purposes of BCC. The solution, as shown in many of the other answers here, was to not include BCC in the headers, and instead only in the list of emails passed to sendmail.
The caveat is that send_message requires a Message object, meaning you'll need to import a class from email.message instead of merely passing strings into sendmail.