Disconnecting internet is a great security advise but i want to check my emails. is that possible to use Ubuntu as host without internet connection while in a guest OS in Virtualbox is able to browse internet ? that way Ubuntu host remain secure
Strictly speaking, no. In order for the guest to have access to the internet, the host needs to provide it to the guest.
If you had the internet connected to the host, you might be able to set up the system to only route traffic to the virtual machine process, but that would still leave your host machine open to the internet.