Lib/smtplib.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/smtplib.py ↗

smtplib provides an SMTP client that speaks RFC 5321. SMTP manages the TCP connection, EHLO/HELO negotiation, AUTH, STARTTLS, and MAIL FROM/RCPT TO/DATA commands. SMTP_SSL connects with TLS from the start.

Map

Lines	Symbol	Role
1-80	Constants, exceptions	SMTPException hierarchy, port constants
81-300	SMTP.__init__, connect, ehlo, helo	Connection and greeting
301-500	starttls, login, auth	TLS upgrade and authentication
501-700	sendmail, send_message	Message sending pipeline
701-850	SMTP.quit, SMTP.close, SMTP.noop	Session management
851-1060	SMTP_SSL, LMTP	SSL and LMTP variants

Reading

Connection and EHLO

# CPython: Lib/smtplib.py:316 SMTP.connect
def connect(self, host='localhost', port=0, source_address=None):
    ...
    self.sock = self._get_socket(host, port, self.timeout)
    ...
    (code, msg) = self.getreply()
    ...
    (code, msg) = self.ehlo_or_helo_if_needed()
    return (code, msg)

After connecting, SMTP reads the greeting, then issues EHLO to discover server extensions. The response populates self.esmtp_features.

sendmail

# CPython: Lib/smtplib.py:826 SMTP.sendmail
def sendmail(self, from_addr, to_addrs, msg, mail_options=(),
             rcpt_options=()):
    ...
    (code, resp) = self.mail(from_addr, mail_options)
    ...
    senderrs = {}
    if isinstance(to_addrs, str):
        to_addrs = [to_addrs]
    for each_addr in to_addrs:
        (code, resp) = self.rcpt(each_addr, rcpt_options)
        if (code != 250) and (code != 251):
            senderrs[each_addr] = (code, resp)
    ...
    (code, resp) = self.data(msg)
    ...
    return senderrs

Issues MAIL FROM, one RCPT TO per address, then DATA. Returns a dict of failed addresses.

STARTTLS

# CPython: Lib/smtplib.py:570 SMTP.starttls
def starttls(self, *, context=None):
    ...
    (resp, reply) = self.docmd("STARTTLS")
    if resp == 220:
        if context is None:
            context = ssl._create_unverified_context()
        self.sock = context.wrap_socket(self.sock, server_hostname=self._host)
        ...
        self.ehlo_resp = self.esmtp_features = {}
        (code, resp) = self.ehlo()
    return (resp, reply)

After STARTTLS succeeds, the EHLO state is reset because the server may advertise different capabilities over TLS.

login and AUTH

login tries AUTH mechanisms in order: LOGIN, PLAIN, CRAM-MD5. The mechanism is selected based on what the server advertised in esmtp_features['auth'].

gopy notes

Status: not yet ported. smtplib depends on socket, ssl, email.message, and base64. The protocol logic is straightforward command/response pairs. A Go port would use net.Conn and crypto/tls.