mailbox does not treat external factories the same

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-09-21.01:37:52.374>
labels = ['type-bug', 'expert-email', '3.10', '3.11']
title = 'mailbox does not treat external factories the same'
updated_at = <Date 2021-06-17.19:43:45.124>
user = 'https://bugs.python.org/bpoaugust'

bugs.python.org fields:

activity = <Date 2021-06-17.19:43:45.124>
actor = 'cryvate'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['email']
creation = <Date 2017-09-21.01:37:52.374>
creator = 'bpoaugust'
dependencies = []
files = ['47159']
hgrepos = []
issue_num = 31538
keywords = ['patch']
message_count = 2.0
messages = ['302667', '302682']
nosy_count = 5.0
nosy_names = ['barry', 'r.david.murray', 'bpoaugust', 'Cryvate', 'cryvate']
pr_nums = ['26776']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31538'
versions = ['Python 3.10', 'Python 3.11']