'Override mail_mail creation to create an entry in mail.mail.statistics'

@api.model
def create(self, values):

    mail = super(MailMail, self).create(values)
    if values.get('statistics_ids'):
        mail_sudo = mail.sudo()
        mail_sudo.statistics_ids.write({'message_id': mail_sudo.message_id, 'state': 'outgoing'})
    return mail
