diff --git a/Lib/email/_header_value_parser.py b/Lib/email/_header_value_parser.py
index f396f4f9b32a9a..f6ff3264c2cb0a 100644
--- a/Lib/email/_header_value_parser.py
+++ b/Lib/email/_header_value_parser.py
@@ -152,7 +152,10 @@ def value(self):
 
     @property
     def all_defects(self):
-        return sum((x.all_defects for x in self), self.defects)
+        if not self:
+            return self.defects
+        return self.defects + [
+            defect for token in self for defect in token.all_defects]
 
     def startswith_fws(self):
         return self[0].startswith_fws()
diff --git a/Lib/email/headerregistry.py b/Lib/email/headerregistry.py
index 48cd85a65ba9f6..329a0cb3c45f83 100644
--- a/Lib/email/headerregistry.py
+++ b/Lib/email/headerregistry.py
@@ -264,6 +264,7 @@ class UnstructuredHeader:
     def parse(cls, value, kwds):
         kwds['parse_tree'] = cls.value_parser(value)
         kwds['decoded'] = str(kwds['parse_tree'])
+        kwds['defects'].extend(kwds['parse_tree'].all_defects)
 
 
 class UniqueUnstructuredHeader(UnstructuredHeader):
diff --git a/Lib/test/test_email/test__header_value_parser.py b/Lib/test/test_email/test__header_value_parser.py
index 26e9249ba9e948..0ee1408e91a657 100644
--- a/Lib/test/test_email/test__header_value_parser.py
+++ b/Lib/test/test_email/test__header_value_parser.py
@@ -7,6 +7,23 @@
 
 class TestTokens(TestEmailBase):
 
+    # TokenList
+
+    def test_all_defects_empty(self):
+        token = parser.TokenList()
+        self.assertIs(token.all_defects, token.defects)
+        token.defects.append(errors.InvalidHeaderDefect())
+        self.assertIs(token.all_defects, token.defects)
+
+    def test_all_defects(self):
+        leaf = parser.ValueTerminal('text\x00', 'utext')
+        leaf.defects.append(errors.NonPrintableDefect('\x00'))
+        token = parser.TokenList([parser.TokenList([leaf]), leaf])
+        token.defects.append(errors.InvalidHeaderDefect())
+        self.assertEqual(token.all_defects, token.defects + leaf.defects * 2)
+        self.assertEqual(len(token.defects), 1)
+        self.assertEqual(len(leaf.defects), 1)
+
     # EWWhiteSpaceTerminal
 
     def test_EWWhiteSpaceTerminal(self):
diff --git a/Lib/test/test_email/test_headerregistry.py b/Lib/test/test_email/test_headerregistry.py
index c118fd5e20d048..b0808b576535ee 100644
--- a/Lib/test/test_email/test_headerregistry.py
+++ b/Lib/test/test_email/test_headerregistry.py
@@ -129,21 +129,67 @@ def test_defects_is_tuple(self):
 @parameterize
 class TestUnstructuredHeader(TestHeaderBase):
 
-    def string_as_value(self,
-                        source,
-                        decoded,
-                        *args):
-        l = len(args)
-        defects = args[0] if l>0 else []
-        header = 'Subject:' + (' ' if source else '')
-        folded = header + (args[1] if l>1 else source) + '\n'
-        h = self.make_header('Subject', source)
-        self.assertEqual(h, decoded)
-        self.assertDefectsEqual(h.defects, defects)
-        self.assertEqual(h.fold(policy=policy.default), folded)
+    def string_as_value(self, source, decoded, defects=(), folded=None):
+        folded = source if folded is None else folded
+        for name in ('Subject', 'X-Test'):
+            with self.subTest(name=name):
+                h = self.make_header(name, source)
+                self.assertEqual(h, decoded)
+                self.assertDefectsEqual(h.defects, defects)
+                prefix = name + ':' + (' ' if source else '')
+                self.assertEqual(h.fold(policy=policy.default),
+                                 prefix + folded + '\n')
 
     string_params = {
 
+        'empty': ('', ''),
+
+        'valid_encoded_word': (
+            '=?utf-8?q?caf=C3=A9?=',
+            'café',
+            []),
+
+        'invalid_base64_padding': (
+            '=?utf-8?b?YQ?=',
+            'a',
+            [errors.InvalidBase64PaddingDefect],
+            'a'),
+
+        'invalid_base64_characters': (
+            '=?utf-8?b?Y!Q==?=',
+            'a',
+            [errors.InvalidBase64CharactersDefect],
+            'a'),
+
+        'undecodable_bytes': (
+            '=?utf-8?q?=FF?=',
+            '\ufffd',
+            [errors.UndecodableBytesDefect] * 2,
+            '=?unknown-8bit?q?=FF?='),
+
+        'unknown_charset': (
+            '=?invalid-charset?q?word?=',
+            'word',
+            [errors.CharsetError],
+            'word'),
+
+        'non_printable_text': (
+            'text\x00value',
+            'text\x00value',
+            [errors.NonPrintableDefect]),
+
+        'combined_defects': (
+            'prefix=?utf-8?b?YQ?=',
+            'prefixa',
+            [errors.InvalidHeaderDefect, errors.InvalidBase64PaddingDefect],
+            'prefixa'),
+
+        'missing_whitespace': (
+            'prefix=?utf-8?q?word?=',
+            'prefixword',
+            [errors.InvalidHeaderDefect],
+            'prefixword'),
+
         'rfc2047_simple_quopri': (
             '=?utf-8?q?this_is_a_test?=',
             'this is a test',
diff --git a/Misc/NEWS.d/next/Library/2026-10-28-18-44-59.gh-issue-158780.J9qR4s.rst b/Misc/NEWS.d/next/Library/2026-10-28-18-44-59.gh-issue-158780.J9qR4s.rst
new file mode 100644
index 00000000000000..db8fed83a3a3d1
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-28-18-44-59.gh-issue-158780.J9qR4s.rst
@@ -0,0 +1,2 @@
+Fix :class:`email.headerregistry.UnstructuredHeader` to include detected
+encoded-word and text defects in its public ``defects`` attribute.