Hi
mm_message_out stores the data produced by mobile sync rules until the mobile device syncs with the server and downloads them, after this they should be removed. So it’s a purley transactional data table.
If you have to million records then yes you have some investigation to complete.
Do you have many mobile users active which are not being used? The mobile dashboard will tell you this. Also do not fall into the trap of assuming the baseline sync rules are fit for all solutions, it is normal process to de-activate some rules and refine the ones which are in use.
Hope this helps.