imaplib module IMAP4.append() unexpected response BAD Command Error

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-04-04.17:40:42.714>
labels = ['extension-modules', 'type-bug']
title = 'imaplib module IMAP4.append() unexpected response BAD Command Error'
updated_at = <Date 2018-04-04.17:40:42.714>
user = 'https://bugs.python.org/yuy'

bugs.python.org fields:

activity = <Date 2018-04-04.17:40:42.714>
actor = 'yuy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2018-04-04.17:40:42.714>
creator = 'yuy'
dependencies = []
files = []
hgrepos = []
issue_num = 33225
keywords = []
message_count = 1.0
messages = ['314945']
nosy_count = 1.0
nosy_names = ['yuy']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33225'
versions = ['Python 3.6']