It doesn't really matter what you put in the To: header as long as the envelope recipient is correctly communicated.
A common trick used by mailing lists is to put the mailing list itself in the To: header:
From: mailing-list@example.edu
To: mailing-list@example.edu
Bcc: actual@example.net, recipients@example.org,
here@example.com, ...
If you pass this to sendmail -t you will get a bounce for the bogus To: address (or a mail loop, if the list ends up sending to itself, and then resending the incoming message to the entire list, etc) but sendmail accepts a list of recipients in a mode where the headers are completely ignored. You could do have this in a file email.txt:
From: me@example.org
To: fred@example.net
Subject: Secret stuff
xyzzy
Now if you do sendmail you@example.com <email.txt the message will be sent to you (only, and not to Fred).
Think of it as a sheet of paper inside an envelope. If the paper inside the envelope says "Santa Claus, North Pole" as the recipient, but you put it in an envelope addressed to "Mr. President, 1600 Pennsylvania Avenue" the message will go to the White House regardless of what it says on the paper inside the sealed envelope.
So in terms of Python code, you could do
msg['Subject'] = 'Subject'
msg['From'] = from_email
msg['To'] = 'noreply@example.org'
# ...
server = SMTP(host, port)
server.ehlo()
server.starttls()
server.login(usr, pwd)
try:
server.sendmail(from_email, my_list, msg.as_string())
and the message will go to the recipients on my_list regardless of the value in the To: header.
On the other hand, if you want an individual message with an individual To: header to be sent for each recipient, you need to modify the To: header in the loop, too.
msg['Subject'] = 'Subject'
msg['From'] = from_email
msg['To'] = 'noreply@example.org'
# ...
server = SMTP(host, port)
server.ehlo()
server.starttls()
server.login(usr, pwd)
for user in my_list do:
try:
msg['To'] = user
server.sendmail(from_email, [user], msg.as_string())
except:
raise HorrorError('Really, you want to raise an exception here')
However, you should understand that if you have multiple recipients in the same domain, their email server will be inundated with virtually identical messages which only differ by the To: header. You could get away with this, especially if the number of recipients in each domain is small, but it is definitely regarded as abusive by some mail administrators, and/or could trigger automated spam filters.
Tangentially, I put in a raise in the except: handler because you really really should not be using a blanket except handler. At the very least, you should capture the error and print out detailed information about what exactly failed; otherwise you are hiding a probably growing number of bugs from yourself.
