Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 13 ■ SMTP
ETRN
STARTTLS
XVERP
8BITMIME
send: 'mail FROM: size=157\r\n'
reply: '250 Ok\r\n'
reply: retcode (250); Msg: Ok
send: 'rcpt TO:\r\n'
reply: '250 Ok\r\n'
reply: retcode (250); Msg: Ok
send: 'data\r\n'
reply: '354 End data with .\r\n'
reply: retcode (354); Msg: End data with .
data: (354, 'End data with .')
send: 'To: jgoerzen@complete.org\r\n
From: foo@example.com\r\n
Subject: Test Message from simple.py\r\n
\r\n
Hello,\r\n
\r\n
This is a test message sent to you from simple.py and smtplib.
\r\n.
\r\n'
reply: '250 Ok: queued as 8094C18C0\r\n'
reply: retcode (250); Msg: Ok: queued as 8094C18C0
data: (250, 'Ok: queued as 8094C18C0')
Message successfully sent to 1 recipient(s)
From this example, you can see the conversation that smtplib is having with the SMTP server over
the network. As you implement code that uses more advanced SMTP features, the details shown here
will be more important, so let’s look at what’s happening.
First, the client (the smtplib library) sends an EHLO command (an “extended” successor to a more
ancient command that was named, more readably, HELO) with your hostname in it. The remote server
responds with its hostname, and lists any optional SMTP features that it supports.
Next, the client sends the mail from command, which states the “envelope sender” e-mail address
and the size of the message. The server at this moment has the opportunity to reject the message (for
example, because it thinks you are a spammer); but in this case, it responds with 250 Ok. (Note that in
this case, the code 250 is what matters; the remaining text is just a human-readable comment and varies
from server to server.)
Then the client sends a rcpt to command, with the “envelope recipient” that we talked so much
about earlier in this chapter; you can finally see that, indeed, it is transmitted separately from the text of
the message itself when using the SMTP protocol. If you were sending the message to more than one
recipient, they would each be listed on the rcpt to line.
Finally, the client sends a data command, transmits the actual message (using verbose carriagereturn-linefeed
line endings, you will note, per the Internet e-mail standard), and finishes the
conversation.
The smtplib module is doing all this automatically for you in this example. In the rest of the chapter,
we will look at how to take more control of the process so you can take advantage of some more
advanced features.
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