python
@@ -9,6 +9,7 @@
 from email.headerregistry import HeaderRegistry as HeaderRegistry
 from email.contentmanager import raw_data_manager
 from email.message import EmailMessage
+from email.errors import NonPrintableDefect
 
 __all__ = [
     'Compat32',
@@ -139,13 +140,22 @@ def header_store_parse(self, name, value):
 
         """
         if hasattr(value, 'name') and value.name.lower() == name.lower():
-            return (name, value)
-        if isinstance(value, str) and len(value.splitlines())>1:
-            # XXX this error message isn't quite right when we use splitlines
-            # (see issue 22233), but I'm not sure what should happen here.
+            header_value = value
+        else:
+            header_value = self.header_factory(name, value)
+
+        np = [
+            non_printable
+            for defect in header_value.defects
+            if isinstance(defect, NonPrintableDefect)
+            for non_printable in defect.non_printables
+        ]
+
+        if linesep_splitter.search(str(header_value)) or '\n' in np or '\r' in np:
             raise ValueError("Header values may not contain linefeed "
                              "or carriage return characters")
-        return (name, self.header_factory(name, value))
+
+        return (name, header_value)
 
     def header_fetch_parse(self, name, value):
         """+

@@ -398,6 +398,60 @@ def test_header_store_parse_rejects_newlines(self):
                           instance.header_store_parse,
                           'From', 'spam\negg@foo.py')
 
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Subject', 'te\nst')
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Subject',
+                          'A 💩 subject=?UTF-8?Q?=0A?=Bcc: injected@example.com')
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Subject',
+                          "Here's an =?UTF-8?Q?embedded_newline=0A?=")
+
+        factory = headerregistry.HeaderRegistry()
+        h = factory('subject', 'te\nst')
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Subject', h)
+
+        h = factory('subject',
+                    "Here's an =?UTF-8?Q?embedded_newline=0A?=")
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Subject', h)
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'From',
+                          "External Sender <sender@them.example.com> =?UTF-8?Q?embedded_newline=0A?=Smuggled-Data: Bad")
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Content-Type',
+                          "text/html; charset=UTF-8=?UTF-8?Q?embedded_newline=0A?=Smuggled-Data: Bad")
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Content-Transfer-Encoding',
+                          "External Sender <sender@them.example.com> =?UTF-8?Q?embedded_newline=0A?=Smuggled-Data: Bad")
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'Message-Id',
+                          "1212334141-321313=?UTF-8?Q?embedded_newline=0A?=Smuggled-Data: Bad")
+
+        self.assertRaises(ValueError,
+                          instance.header_store_parse,
+                          'content-disposition',
+                          "inline=?UTF-8?Q?embedded_newline=0A?=Smuggled-Data: Bad")
+
+        # date and mime headers don't have embedded newlines, nor errors
+
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2024-07-15-21-43-38.gh-issue-121650.A0FkCh.rst b/Misc/NEWS.d/next/Library/2024-07-15-21-43-38.gh-issue-121650.A0FkCh.rst
@@ -0,0 +1,3 @@
+Fix :mod:`email.policy` to properly disallow newlines and carriage returns
+in email headers, both at the end of the header, and encoded entities that are
+resolved to newlines and carriage returns.