Refs #36664 -- Updated SMTP EmailBackend tests for Python 3.15.

Versions of Python prior to 3.15 would incorrectly encode non-ASCII
email addresses using rfc2047, resulting in undeliverable email. The
SMTP EmailBackend detects and prevents that (#35713). Python 3.15 fixes
that behavior (CPython issue gh-122476).

Updated test_rejects_non_ascii_local_part() to feature-detect the fix
(in case it is backported) and check for a representative section of
the Python error message if so; otherwise test for the SMTP EmailBackend
workaround.

Updated comments to clarify need and requirement.

2 files changed, 24 insertions, 11 deletions

diff --git a/django/core/mail/backends/smtp.py b/django/core/mail/backends/smtp.py
index e7cca2c01d..84154dd1c0 100644
--- a/django/core/mail/backends/smtp.py
+++ b/django/core/mail/backends/smtp.py
@@ -217,8 +217,10 @@ class EmailBackend(BaseEmailBackend):
         # Django allows local mailboxes like "From: webmaster" (#15042).
         defects.discard("addr-spec local part with no domain")
         if not force_ascii:
-            # Non-ASCII local-part is valid with SMTPUTF8. Remove once
-            # https://github.com/python/cpython/issues/81074 is fixed.
+            # PY315: Non-ASCII local-part is valid with SMTPUTF8. This check
+            # can be removed once the minimum supported Python version is 3.15
+            # (so the fix for https://github.com/python/cpython/issues/81074
+            # is in all supported versions).
             defects.discard("local-part contains non-ASCII characters)")
         if defects:
             raise ValueError(f"Invalid address {address!r}: {'; '.join(defects)}")
diff --git a/tests/mail/test_backends.py b/tests/mail/test_backends.py
index eec7568a2d..0782b18afe 100644
--- a/tests/mail/test_backends.py
+++ b/tests/mail/test_backends.py
@@ -5,6 +5,8 @@ import socket
 import ssl
 import sys
 import tempfile
+from email.errors import HeaderWriteError
+from email.message import EmailMessage as PyEmailMessage
 from io import StringIO
 from pathlib import Path
 from smtplib import SMTPException
@@ -1093,17 +1095,26 @@ class SMTPBackendTests(SharedEmailBackendTests, SMTPBackendTestsBase):
         )
 
     def test_rejects_non_ascii_local_part(self):
-        """
-        The SMTP EmailBackend does not currently support non-ASCII local-parts.
-        (That would require using the RFC 6532 SMTPUTF8 extension.) #35713.
-        """
+        # The SMTP EmailBackend must work around invalid email encoding caused
+        # by https://github.com/python/cpython/issues/122476 (#35713).
+        try:
+            # Detect fix for CPython issue gh-122476.
+            message = PyEmailMessage()
+            message["To"] = "nø@example.dk"
+            message.as_bytes()
+        except HeaderWriteError:
+            # PY315: Error from Python email generator.
+            msg = "Non-ASCII local-part 'nø' is invalid"
+        else:
+            # Python <=3.14: Error from smtp.EmailBackend.prep_address().
+            msg = (
+                "Invalid address 'nø@example.dk': local-part contains "
+                "non-ASCII characters"
+            )
+
         backend = self.create_backend()
-        backend.connection = mock.Mock(spec=object())
         email = EmailMessage(to=["nø@example.dk"])
-        with self.assertRaisesMessage(
-            ValueError,
-            "Invalid address 'nø@example.dk': local-part contains non-ASCII characters",
-        ):
+        with self.assertRaisesMessage((ValueError, HeaderWriteError), msg):
             backend.send_messages([email])
 
     def test_prep_address_without_force_ascii(self):