For a message to be considered as a private message,
the message must have:
- `no_auto_thread` set to `False`
in other words, it must not be attached to a thread
- no `model` or `res_id` must be set on the message
meaning the message is not attached to a specific
record thread.
See `_get_message_id` in `mail/models/mail_message.py`.
Before this revision, the mails sent from
`Discuss` > `Send mail` were not regarded
as private messages,
but as a kind of mass mailing messages,
with a `reply-to` wrongly set.
Any attempt to reply to these mails
failed with a delivery failure notification.
This was because the `message-id` of these messages
contained `reply-to`, and such message-ids are
treated as a special case in the messages routing
(basically, the match on the mail references is disabled
for these messages containing `reply-to` in
their message-id).
opw-669241
Denis Ledoux
002722b967