When using the composer in mass mail mode, no explicit 'message_type' is given
to created 'mail.mail' records. By default those are set to 'email'.
In this commit we make it explicit, as it may change in a near future. This
type is not really 'notification', 'comment' or 'user_notification', so using
'email' is fine currently. It is used more for incoming emails but we don't
distinguish outgoing emails currently.
Task-2710804 (Mail: Clean MailThread API)
Part-of: odoo/odoo#100137
Thibault Delavallée
c1f1333d88