Bug report

Bug description:

Thesendmail function in smtplib does a good job of handling 421 responses from SMTP servers but is missing one case. This may lead to SMTP client connections not being closed properly.

Background

SMTP servers can send 421 responses whenever they need to shut down the connection (RF 5321 3.8). Clients should take care of closing the connection when receiving a 421.

Details

Thesendmail function combines the low-level SMTP commands into a full session for sending some text content via SMTP. The function needs to handle 421 responses, as any single command may return a 421 response. The function already handles almost all cases but is missing one for the call of thedata function, which does its own response code checking and mayraise an exception.

Test to reproduce

The test was derived from the correspondingrosetta-test SMTP test suite and adapted to the smtplib test infrastructure.

deftest_421_during_data_cmd(self):smtp=smtplib.SMTP(HOST,self.port,local_hostname='localhost',timeout=support.LOOPBACK_TIMEOUT)smtp.noop()self.serv._SMTPchannel.data_response='421 closing'withself.assertRaises(smtplib.SMTPDataError):smtp.sendmail('John@foo.org', ['Sally@foo.org'],'test message')self.assertIsNone(smtp.sock)self.assertEqual(self.serv._SMTPchannel.rcpt_count,0)

CPython versions tested on:

CPython main branch, 3.10, 3.12

Operating systems tested on:

Linux

Linked PRs

• gh-132796: Closes SMTP connection on 421 during data in sendmail #132797