gh-136686 Fix UnicodeEncodeError in email.generator.write when encoding multipart/related emails using utf8 policy

Lib/email/generator.py

@@ -417,7 +417,11 @@ class BytesGenerator(Generator):
     """
 
     def write(self, s):
-        self._fp.write(s.encode('ascii', 'surrogateescape'))
+        if getattr(self.policy, "utf8", False):
+            encoded = s.encode('utf-8', 'surrogateescape')
+        else:
+            encoded = s.encode('ascii', 'surrogateescape')
+        self._fp.write(encoded)
 
     def _new_buffer(self):
         return BytesIO()

Lib/test/test_email/test_generator.py

@@ -471,6 +471,20 @@ def test_smtp_policy(self):
         g = BytesGenerator(s, policy=policy.SMTP)
         g.flatten(msg)
         self.assertEqual(s.getvalue(), expected)
+    def test_utf8_round_trip_preserves_unicode_body(self):
+        msg = EmailMessage()
+        msg['From'] = "Páolo <fő[email protected]>"
+        msg['To'] = 'Dinsdale'
+        msg['Subject'] = 'Nudge nudge, wink, wink \u1F609'
+        msg.set_content("oh là là, know what I mean, know what I mean?")
+        s = io.BytesIO()
+        g = BytesGenerator(s, policy=policy.SMTPUTF8)
+        g.flatten(msg)
+        out = s.getvalue()
+        parsed = message_from_bytes(out, policy=policy.default.clone(utf8=True))
+
+        self.assertEqual(parsed["Subject"], 'Nudge nudge, wink, wink \u1F609')
+        self.assertEqual(parsed.get_body().get_content(), "oh là là, know what I mean, know what I mean?\r\n")
 
 
 if __name__ == '__main__':

Misc/NEWS.d/next/Library/2025-07-19-22-54-48.gh-issue-136686.fc2Zl0.rst

@@ -0,0 +1,2 @@
+Fixed a bug where emails with ``multipart/related`` content did not round-trip
+correctly through ``email.parser.parsebytes()`` and ``as_bytes()``.