Especially in the context of a small business of just 6 folks... yes, I think keeping tabs on your email is understandable.

There are 10 people that work for my company... half of them engineers like myself. It's expected that we aren't completely disconnected on vacation... but at least available if a true emergency crops up that needs our niche expertise.

That said, my boss is awesome and encourages healthy work-life separation, so stuff has to really escalate before you're expected to jump in...