Mike, have you considered becoming a Zimbra Hosting Provider?
We initially chose Zimbra for our own internal use, but two smaller clients facing Exchange upgrades saw us using it and wanted a demo. They decided they wanted Zimbra instead of Exchange, we ran the numbers and it was better for everyone if we became a Hosting Provider. So we did.
We have plenty of clients for whom Exchange is more appropriate than Zimbra (usually due to third-party app integration with Exchange), but becoming a Zimbra Hosting Provider might help you better serve your smaller clients.
Hope that helps,