这是一个老问题。我发布新答案的主要原因是解释如何用Python 3.6+中的现代电子邮件库解决问题，以及它与旧版本的区别;但首先，让我们回顾一下Anony-Mousse在2012年的回答。
SMTP根本不关心头文件中有什么。您传递给sendmail方法的收件人列表实际上决定了消息将被传递到哪里。
在SMTP术语中，这称为消息的信封。在协议级别上，您连接到服务器，然后告诉它消息来自谁(MAIL from: SMTP动词)以及将消息发送给谁(RCPT to:)，然后分别传输消息本身(DATA)，其标题和正文作为一个斜向字符串blob。
现代smtplib通过提供send_message方法简化了Python方面的工作，该方法实际发送给消息头中指定的收件人。
现代电子邮件库提供了一个EmailMessage对象，它取代了所有不同的MIME类型，过去您必须使用这些MIME类型从较小的部分组装消息。您可以添加附件，而不需要单独构造它们，如果需要，还可以构建各种更复杂的多部分结构，但通常不必这样做。只需创建一条消息并填充您想要的部分。
注意，下面有大量注释;总的来说，新的EmailMessage API比旧的API更简洁、更通用。
from email.message import EmailMessage
msg = EmailMessage()
# This example uses explicit strings to emphasize that
# that's what these header eventually get turned into
msg["From"] = "me@example.org"
msg["To"] = "main.recipient@example.net, other.main.recipient@example.org"
msg["Cc"] = "secondary@example.com, tertiary@example.eu"
msg["Bcc"] = "invisible@example.int, undisclosed@example.org.au"
msg["Subject"] = "Hello from the other side"
msg.set_content("This is the main text/plain message.")
# You can put an HTML body instead by adding a subtype string argument "html"
# msg.set_content("<p>This is the main text/html message.</p>", "html")
# You can add attachments of various types as you see fit;
# if there are no other parts, the message will be a simple
# text/plain or text/html, but Python will change it into a
# suitable multipart/related or etc if you add more parts
with open("image.png", "rb") as picture:
msg.add_attachment(picture.read(), maintype="image", subtype="png")
# Which port to use etc depends on the mail server.
# Traditionally, port 25 is SMTP, but modern SMTP MSA submission uses 587.
# Some servers accept encrypted SMTP_SSL on port 465.
# Here, we use SMTP instead of SMTP_SSL, but pivot to encrypted
# traffic with STARTTLS after the initial handshake.
with smtplib.SMTP("smtp.example.org", 587) as server:
# Some servers insist on this, others are more lenient ...
# It is technically required by ESMTP, so let's do it
# (If you use server.login() Python will perform an EHLO first
# if you haven't done that already, but let's cover all bases)
server.ehlo()
# Whether or not to use STARTTLS depends on the mail server
server.starttls()
# Bewilderingly, some servers require a second EHLO after STARTTLS!
server.ehlo()
# Login is the norm rather than the exception these days
# but if you are connecting to a local mail server which is
# not on the public internet, this might not be useful or even possible
server.login("me.myself@example.org", "xyzzy")
# Finally, send the message
server.send_message(msg)
Bcc:标题的最终可见性取决于邮件服务器。如果你真的想确保收件人之间是不可见的，也许根本就不要使用Bcc:头，并且在信封中单独列举信封中的收件人，就像你以前在sendmail中必须做的那样(send_message也允许你这样做，但如果你只是想发送给头中指定的收件人，你就不必这样做)。
This obviously sends a single message to all recipients in one go. That is generally what you should be doing if you are sending the same message to a lot of people. However, if each message is unique, you will need to loop over the recipients and create and send a new message for each. (Merely wishing to put the recipient's name and address in the To: header is probably not enough to warrant sending many more messages than required, but of course, sometimes you have unique content for each recipient in the body, too.)
