gh-73065: Add Date header if missing in smtplib send_message

Doc/library/smtplib.rst

@@ -510,12 +510,17 @@ An :class:`SMTP` instance has the following methods:
  specified in :rfc:`5322`\: *from_addr* is set to the :mailheader:`Sender`
  field if it is present, and otherwise to the :mailheader:`From` field.
  *to_addrs* combines the values (if any) of the :mailheader:`To`,
- :mailheader:`Cc`, and :mailheader:`Bcc` fields from *msg*. If exactly one
- set of :mailheader:`Resent-*` headers appear in the message, the regular
- headers are ignored and the :mailheader:`Resent-*` headers are used instead.
- If the message contains more than one set of :mailheader:`Resent-*` headers,
- a :exc:`ValueError` is raised, since there is no way to unambiguously detect
- the most recent set of :mailheader:`Resent-` headers.
+ :mailheader:`Cc`, and :mailheader:`Bcc` fields from *msg*. If there's no
+ :mailheader:`Date` header inside the message, ``send_message`` will add one to the data.
+ If exactly one set of :mailheader:`Resent-*` headers appear in the message,
+ the regular headers are ignored and the :mailheader:`Resent-*` headers are
+ used instead. If the message contains more than one set of
+ :mailheader:`Resent-*` headers, a :exc:`ValueError` is raised, since there
+ is no way to unambiguously detect the most recent set of
+ :mailheader:`Resent-` headers.
+
+ .. versionchanged:: next
+ Support to add :mailheader:`Date` header to the message if one does not exist.
 
  ``send_message`` serializes *msg* using
  :class:`~email.generator.BytesGenerator` with ``\r\n`` as the *linesep*, and

Lib/smtplib.py

@@ -935,6 +935,11 @@ def send_message(self, msg, from_addr=None, to_addrs=None,
  header_prefix = 'Resent-'
  else:
  raise ValueError("message has more than one 'Resent-' header block")
+
+ # RFC 5322 section 3.6, 4th Paragraph
+ if msg.get('Date', None) is None:
+ # localtime: RFC 5322 section 3.3, 4th Paragraph
+ msg['Date'] = email.utils.formatdate(localtime=True)
  if from_addr is None:
  # Prefer the sender field per RFC 2822:3.6.2.
  from_addr = (msg[header_prefix + 'Sender']

Lib/test/test_smtplib.py

@@ -1467,6 +1467,7 @@ def test_send_unicode_with_SMTPUTF8_via_low_level_API(self):
  self.assertIn('SMTPUTF8', self.serv.last_mail_options)
  self.assertEqual(self.serv.last_rcpt_options, [])
 
+ @support.run_with_tz('UTC-02')
  def test_send_message_uses_smtputf8_if_addrs_non_ascii(self):
  msg = EmailMessage()
  msg['From'] = "Páolo <főo@bar.com>"
@@ -1477,24 +1478,35 @@ def test_send_message_uses_smtputf8_if_addrs_non_ascii(self):
  msg.set_content("oh là là, know what I mean, know what I mean?\n\n")
  # XXX smtpd converts received /r/n to /n, so we can't easily test that
  # we are successfully sending /r/n :(.
+ smtp = smtplib.SMTP(
+ HOST, self.port, local_hostname='localhost',
+ timeout=support.LOOPBACK_TIMEOUT)
+ self.addCleanup(smtp.close)
+ self.assertEqual(smtp.send_message(msg),{})
+
+ last_message = self.serv.last_message.decode()
+ date = email.message_from_string(last_message)['Date']
+ # asserts RFC 5322 section 3.3 4th Paragraph
+ self.assertEqual(
+ email.utils.parsedate_to_datetime(date).tzname(),
+ "UTC+02:00"
+ )
+
  expected = textwrap.dedent("""\
  From: Páolo <főo@bar.com>
  To: Dinsdale
  Subject: Nudge nudge, wink, wink \u1F609
  Content-Type: text/plain; charset="utf-8"
  Content-Transfer-Encoding: 8bit
  MIME-Version: 1.0
+ Date:{}
 
  oh là là, know what I mean, know what I mean?
- """)
- smtp = smtplib.SMTP(
- HOST, self.port, local_hostname='localhost',
- timeout=support.LOOPBACK_TIMEOUT)
- self.addCleanup(smtp.close)
- self.assertEqual(smtp.send_message(msg),{})
+ """.format(date))
+
  self.assertEqual(self.serv.last_mailfrom, 'főo@bar.com')
  self.assertEqual(self.serv.last_rcpttos, ['Dinsdale'])
- self.assertEqual(self.serv.last_message.decode(), expected)
+ self.assertEqual(last_message, expected)
  self.assertIn('BODY=8BITMIME', self.serv.last_mail_options)
  self.assertIn('SMTPUTF8', self.serv.last_mail_options)
  self.assertEqual(self.serv.last_rcpt_options, [])

Misc/NEWS.d/next/Library/2025-07-20-09-51-25.bpo-28879.SLDgcy.rst

@@ -0,0 +1,2 @@
+Fix ``smtplib.send_message`` to add a ``Date`` header if it is missing as
+per :rfc:`5322`.