After much research I came to some conclusions on the options
a/ 3rd party applications: The only one avaliable for E61 is Profmail. Yes you can set the pull interval for any time. Problem is that you can't set your business works hours like the default E61 mail client. I only want mail pulled from 7-7pm M-F, once at home I check my mail from my computer. Secondly cos Profmail isn't directly connected to the OS, it doesn't share the alert settings, for instance if you are in meeting mode, it doesn't have vibrate alert.
b/ Push email: The problem with push email is you can't set business hours, so they keep pushing mail even when you don't want it.
In the end I have to live with the 30min interval. I don't know who came up with the idea in Symbian and why they can't change this.