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

Doc/library/smtplib.rst

@@ -534,12 +534,17 @@ 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

@@ -949,6 +949,11 @@ 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 'Date' not in msg:
+            # 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 5322 section 3.6.2.
             from_addr = (msg[header_prefix + 'Sender']

Lib/test/test_smtplib.py

@@ -1504,6 +1504,7 @@ 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>"
@@ -1514,24 +1515,35 @@ 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 :(.
-        expected = textwrap.dedent("""\
+        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(f"""\
             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: {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), {})
+
         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`.