Can you try this, instead of using 'zmmailboxdctl start', use 'zmstorectl start'. See if it stays up then.
I believe that zmstorectl start not only starts the mailboxd, but also the mysql db that is attached to it. Without the db running, I imagine mailboxd may be failing because of it.
If 'zmstorectl start' fixes it so it seems stable, most likely you are having the same issue I was having with it.
In which case, you would want to do the following command to fix it:
Code:
zmprov ms ns.inner-werx.com +zimbraServiceEnabled mailbox
Hope that helps.
Aaron