Further, under Python 3.8 the issue is not fully solved, as other identification headers are still being folded in a non-RFC-conformant manner (see OP for RFC references). This was indicated on the original PR by the author: https://github.com/python/cpython/pull/13397#issuecomment-493618544

It is less severe of a problem than for Message-ID, but still means that MTA/MUA may fail to recognize the threading structure because identifiers are lost.

Is it better to open a new issue for this?


# Example on 3.8.2: the `In-Reply-To` header is RFC2047-folded.

Python 3.8.2 (default, Jul 16 2020, 14:00:26) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import email.message
>>> import email.policy
>>> msg = email.message.EmailMessage(policy=email.policy.SMTP)
>>> msg['Message-Id'] = '<1596730490.324691772460938-example-30661-some.reference@test-123.example.com">929227342217024.1596730490.324691772460938-example-30661-some.reference@test-123.example.com>'
>>> msg['In-Reply-To'] = '<1596730568.324691772460444-another-30661-parent.reference@test-123.example.com">92922734221723.1596730568.324691772460444-another-30661-parent.reference@test-123.example.com>'
>>> print(msg.as_string())
Message-Id: <1596730490.324691772460938-example-30661-some.reference@test-123.example.com">929227342217024.1596730490.324691772460938-example-30661-some.reference@test-123.example.com>
In-Reply-To: =?utf-8?q?=3C92922734221723=2E1596730568=2E324691772460444-anot?=
 =?utf-8?q?her-30661-parent=2Ereference=40test-123=2Eexample=2Ecom=3E?=