I'd say it depends on where you want to devote your time. If you have the desire and the time to investigate Zimbra and Linux at large, then sure it'd be a great learning experience and beneficial; there's nothing stopping you from being self taught.
However, if you'd like to dedicate your time and energies to the new business, then I'd recommend using a hosted service or if it's currently working ok for you - sticking with what you have. Come back to building your own enterprise email solution once you've built your enterprise
Good luck with everything.