I tried the test case on master branch. I ran the test case on 1GB RAM Linux based digitalocean droplet to have the script killed. Please find the results as below : # Python build ➜ cpython git:(master) ✗ ./python Python 3.8.0a0 (heads/bpo33095-add-reference:9d49f85, Jun 17 2018, 07:22:33) [GCC 5.4.0 20160609] on linux Type "help", "copyright", "credits" or "license" for more information. >>> # Test case ➜ cpython git:(master) ✗ cat foo.py from email.message import EmailMessage from email.policy import default policy = default # max_line_length = 78 msg = EmailMessage() msg["Subject"] = "á"*100 policy.fold("Subject", msg["Subject"]) # Test case execution ➜ cpython git:(master) ✗ time ./python foo.py [2] 13637 killed ./python foo.py ./python foo.py 387.36s user 3.85s system 90% cpu 7:11.94 total # I tried to do Ctrl + C after 2 minutes to stop and the stack trace is as below : ➜ cpython git:(master) ✗ time ./python foo.py ^CTraceback (most recent call last): File "foo.py", line 7, in <module> policy.fold("Subject", msg["Subject"]) File "/root/cpython/Lib/email/policy.py", line 183, in fold return self._fold(name, value, refold_binary=True) File "/root/cpython/Lib/email/policy.py", line 205, in _fold return value.fold(policy=self) File "/root/cpython/Lib/email/headerregistry.py", line 258, in fold return header.fold(policy=policy) File "/root/cpython/Lib/email/_header_value_parser.py", line 144, in fold return _refold_parse_tree(self, policy=policy) File "/root/cpython/Lib/email/_header_value_parser.py", line 2650, in _refold_parse_tree part.ew_combine_allowed, charset) File "/root/cpython/Lib/email/_header_value_parser.py", line 2728, in _fold_as_ew ew = _ew.encode(first_part, charset=encode_as) File "/root/cpython/Lib/email/_encoded_words.py", line 226, in encode qlen = _cte_encode_length['q'](bstring) File "/root/cpython/Lib/email/_encoded_words.py", line 93, in len_q return sum(len(_q_byte_map[x]) for x in bstring) File "/root/cpython/Lib/email/_encoded_words.py", line 93, in <genexpr> return sum(len(_q_byte_map[x]) for x in bstring) KeyboardInterrupt ./python foo.py 131.41s user 0.43s system 98% cpu 2:13.89 total Thanks