gh-142006: Fix HeaderWriteError in email.policy.default caused by extra newline

Lib/email/_header_value_parser.py

@@ -2792,6 +2792,10 @@ def _steal_trailing_WSP_if_exists(lines):
     if lines and lines[-1] and lines[-1][-1] in WSP:
         wsp = lines[-1][-1]
         lines[-1] = lines[-1][:-1]
+        # FIX: If the line is now empty (it was only a space), remove it entirely
+        # to prevent creating an empty line (double newline) in the output.
+        if not lines[-1]:
+            lines.pop()
     return wsp
 
 def _refold_parse_tree(parse_tree, *, policy):

Lib/test/test_email/test_policy.py

@@ -165,6 +165,20 @@ def test_fold_zero_max_line_length(self):
         self.assertEqual(p1.fold('Subject', msg['Subject']), expected)
         self.assertEqual(p2.fold('Subject', msg['Subject']), expected)
 
+    def test_fold_address_list_with_stolen_whitespace(self):
+        long_address = (
+            "loooooooooooooooooooooooooooooooooooong@dooooooooooooomainname.example.com, "
+            "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.example.com"
+        )
+        msg = email.message_from_string(f"To: {long_address}\n\n", policy=email.policy.default)
+
+        # This should not raise HeaderWriteError
+        output = msg.as_string()
+        # Verify the output format is correct and clean
+        self.assertIn(long_address.split(',')[0], output)
+        # Ensure header section doesn't have double newlines
+        self.assertNotIn('\n\n', output.split('\n\n')[0])
+
     def test_register_defect(self):
         class Dummy:
             def __init__(self):

Misc/NEWS.d/next/Library/2025-11-27-10-49-13.gh-issue-142006.nzJDG5.rst

@@ -0,0 +1 @@
+Fix :exc:`email.errors.HeaderWriteError` in :mod:`email.policy.default` which created double newlines when folding long headers with list separators.