'Return a specific reply_to: alias of the document through
message_get_reply_to or take the email_from'

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

    (model, res_id, email_from) = (values.get('model', self._context.get('default_model')), values.get('res_id', self._context.get('default_res_id')), values.get('email_from'))
    if model:
        return self.env[model].message_get_reply_to([res_id], default=email_from)[res_id]
    else:
        return self.env['mail.thread'].message_get_reply_to([None], default=email_from)[None]
