use of UNSET in AnymailMessage breaks compatibility with django-mailer?

[ewingrj]

Reporting an error? It's helpful to know:

• Anymail version - 6.0
• ESP (Mailgun, SendGrid, etc.)
• Your ANYMAIL settings (change secrets to "redacted")
• Versions of Django, requests, python
• Exact error message and/or stack trace

Traceback (most recent call last):
  File "./manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/django/core/management/base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/django/core/management/base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/mailer/management/commands/send_mail.py", line 25, in handle
    send_all()
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/mailer/engine.py", line 155, in send_all
    email.send()
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/django/core/mail/message.py", line 291, in send
    return self.get_connection(fail_silently).send_messages([self])
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/base.py", line 95, in send_messages
    sent = self._send(message)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/base_requests.py", line 60, in _send
    return super(AnymailRequestsBackend, self)._send(message)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/base.py", line 124, in _send
    payload = self.build_message_payload(message, self.send_defaults)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/mailjet.py", line 29, in build_message_payload
    return MailjetPayload(message, defaults, self)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/mailjet.py", line 90, in __init__
    auth=auth, headers=http_headers, *args, **kwargs)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/base_requests.py", line 148, in __init__
    super(RequestsPayload, self).__init__(message, defaults, backend)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/backends/base.py", line 281, in __init__
    value = converter(value, field=attr)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/utils.py", line 181, in parse_single_address
    parsed = parse_address_list([address], field=field)
  File "/Users/rjewing/.pyenv/versions/crowdly/lib/python3.7/site-packages/anymail/utils.py", line 168, in parse_address_list
    raise AnymailInvalidAddress(errmsg)
anymail.exceptions.AnymailInvalidAddress: Invalid email address 'object object at 0x10f639f80' parsed from '<object object at 0x10f639f80>' in `envelope_sender`.

• Any other relevant code and settings (e.g., for problems
  sending, your code that sends the message)

django-mailer is mentioned in the docs, so it appears it should be supported, however I am getting the above error when I attempt to use AnymailMessage. It looks like this is b/c you check for envelope_sender is not UNSET however the UNSET object is not the same as when the message was created b/c django-mailer pickles & unpickles the email in a different process (cron job)

It works correctly when setting anymail attributes on django's EmailMultiAlternatives

[medmunds]

Good find—this is definitely a bug. Thanks for tracking this down to UNSET's identity not surviving pickling.

[medmunds]

Fix released in Anymail 6.0.1