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

Lib/smtplib.py

@@ -886,7 +886,14 @@ def sendmail(self, from_addr, to_addrs, msg, mail_options=(),
             # the server refused all our recipients
             self._rset()
             raise SMTPRecipientsRefused(senderrs)
-        (code, resp) = self.data(msg)
+        try:
+            (code, resp) = self.data(msg)
+        except SMTPDataError as ex:
+            if ex.smtp_code == 421:
+                self.close()
+            else:
+                self._rset()
+            raise
         if code != 250:
             if code == 421:
                 self.close()

Lib/test/test_smtplib.py

@@ -1259,6 +1259,10 @@ def test__rest_from_mail_cmd(self):
             smtp.sendmail('John', 'Sally', 'test message')
         self.assertIsNone(smtp.sock)
 
+    # The following 421 response tests for sendmail ensure that sendmail handles
+    # 421 respones correctly by closing the connection. sendmail has to take
+    # care of this, as it wraps a mail transaction for users.
+
     # Issue 5713: make sure close, not rset, is called if we get a 421 error
     def test_421_from_mail_cmd(self):
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost',
@@ -1297,6 +1301,16 @@ def found_terminator(self):
         self.assertIsNone(smtp.sock)
         self.assertEqual(self.serv._SMTPchannel.rcpt_count, 0)
 
+    def test_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'
+        with self.assertRaises(smtplib.SMTPDataError):
+            smtp.sendmail('[email protected]', ['[email protected]'], 'test message')
+        self.assertIsNone(smtp.sock)
+        self.assertEqual(self.serv._SMTPchannel.rcpt_count, 0)
+
     def test_smtputf8_NotSupportedError_if_no_server_support(self):
         smtp = smtplib.SMTP(
             HOST, self.port, local_hostname='localhost',

Misc/NEWS.d/next/Library/2025-05-20-15-39-55.gh-issue-132796.UJXDBR.rst

@@ -0,0 +1,3 @@
+Improve handling of 421 SMTP responses in :meth:`smtplib.SMTP.sendmail` by
+covering a previously missing case in which 421 occurs while sending a DATA
+command.