Forwarding an email with python smtplib

I'm trying to put together a script that automatically forwards certain emails that match a specific criteria to another email.

I've got the downloading and parsing of messages using imaplib and email working, but I can't figure out how to forward an entire email to another address. Do I need to build a new message from scratch, or can I somehow modify the old one and re-send it?

Here's what I have so far (client is an imaplib.IMAP4 connection, and id is a message ID):

import smtplib, imaplib

smtp = smtplib.SMTP(host, smtp_port)
smtp.login(user, passw)

client = imaplib.IMAP4(host)
client.login(user, passw)
client.select('INBOX')

status, data = client.fetch(id, '(RFC822)')
email_body = data[0][1]
mail = email.message_from_string(email_body)

# ...Process message...

# This doesn't work
forward = email.message.Message()
forward.set_payload(mail.get_payload())
forward['From'] = 'source.email.address@domain.com'
forward['To'] = 'my.开发者_如何学编程email.address@gmail.com'

smtp.sendmail(user, ['my.email.address@gmail.com'], forward.as_string())

I'm sure there's something slightly more complicated I need to be doing with regard to the MIME content of the message. Surely there's some simple way of just forwarding the entire message though?

# This doesn't work either, it just freezes...?
mail['From'] = 'source.email.address@domain.com'
mail['To'] = 'my.email.address@gmail.com'
smtp.sendmail(user, ['my.email.address@gmail.com'], mail.as_string())

I think the part you had wrong was how to replace the headers in the message, and the fact that you don't need to make a copy of the message, you can just operate directly on it after creating it from the raw data you fetched from the IMAP server.

You did omit some detail so here's my complete solution with all details spelled out. Note that I'm putting the SMTP connection in STARTTLS mode since I need that and note that I've separated the IMAP phase and the SMTP phase from each other. Maybe you thought that altering the message would somehow alter it on the IMAP server? If you did, this should show you clearly that that doesn't happen.

import smtplib, imaplib, email

imap_host = "mail.example.com"
smtp_host = "mail.example.com"
smtp_port = 587
user = "xyz"
passwd = "xyz"
msgid = 7
from_addr = "from.me@example.com"
to_addr = "to.you@example.com"

# open IMAP connection and fetch message with id msgid
# store message data in email_data
client = imaplib.IMAP4(imap_host)
client.login(user, passwd)
client.select('INBOX')
status, data = client.fetch(msgid, "(RFC822)")
email_data = data[0][1]
client.close()
client.logout()

# create a Message instance from the email data
message = email.message_from_string(email_data)

# replace headers (could do other processing here)
message.replace_header("From", from_addr)
message.replace_header("To", to_addr)

# open authenticated SMTP connection and send message with
# specified envelope from and to addresses
smtp = smtplib.SMTP(smtp_host, smtp_port)
smtp.starttls()
smtp.login(user, passwd)
smtp.sendmail(from_addr, to_addr, message.as_string())
smtp.quit()

Hope this helps even if this answer comes quite late.