gh-159304: Fix AttributeError in email get_body() for nested malformed multipart (#159306)

MIMEPart._find_body() incorrectly checked self.is_multipart() instead of
part.is_multipart() when recursing.

Lib/email/message.py

diff --git a/Lib/email/message.py b/Lib/email/message.py
index 64e6c1e..093b901 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -1026,7 +1026,7 @@ def _find_body(self, part, preferencelist):
             if subtype in preferencelist:
                 yield (preferencelist.index(subtype), part)
             return
-        if maintype != 'multipart' or not self.is_multipart():
+        if maintype != 'multipart' or not part.is_multipart():
             return
         if subtype != 'related':
             for subpart in part.iter_parts():

Lib/test/test_email/test_message.py

diff --git a/Lib/test/test_email/test_message.py b/Lib/test/test_email/test_message.py
index 56ad446..974b0ce 100644
--- a/Lib/test/test_email/test_message.py
+++ b/Lib/test/test_email/test_message.py
@@ -1081,6 +1081,25 @@ def test_get_body_malformed(self):
         # AttributeError: 'str' object has no attribute 'is_attachment'
         m.get_body()
 
+    def test_get_body_malformed_nested_multipart(self):
+        # gh-159304: a nested multipart part without a boundary has a
+        # string payload, which get_body() must not iterate over.
+        msg = textwrap.dedent("""\
+            From: sender@example.com
+            To: receiver@example.com
+            Subject: Malformed
+            Content-Type: multipart/related; boundary=OUTER
+
+            --OUTER
+            Content-Type: multipart/related
+
+            raw body no boundary
+            --OUTER--
+            """)
+        m = self._str_msg(msg)
+        self.assertIsNone(m.get_body(('plain', 'html')))
+        self.assertIs(m.get_body(('related',)), m)
+
     def test_get_bytes_payload_with_quoted_printable_encoding(self):
         # We use a memoryview to avoid directly changing the private payload
         # and to prevent using the dedicated paths for string or bytes objects.

Misc/NEWS.d/next/Library/2026-10-13-12-00-00.gh-issue-159304.mFbYq2.rst

diff --git a/Misc/NEWS.d/next/Library/2026-10-13-12-00-00.gh-issue-159304.mFbYq2.rst b/Misc/NEWS.d/next/Library/2026-10-13-12-00-00.gh-issue-159304.mFbYq2.rst
new file mode 100644
index 0000000..8f5b61e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-13-12-00-00.gh-issue-159304.mFbYq2.rst
@@ -0,0 +1,2 @@
+Fix :exc:`AttributeError` in :meth:`email.message.EmailMessage.get_body` when a
+nested ``multipart`` part is malformed and its payload is a string.