Can I open email by clicking on notification?

The purpose of the notifications is, well, to notify you. Nothing else. Once you receive the notification you go to the corresponding indicator (in the top right) and open it. In this case you would go to the little envelope icon.

You don't need to put the mouse over it: just read it and make use of that information. If you put the mouse over the notification bubble, it blurs so you can easily read or interact with what's under it.

Another side effect of putting the mouse over the notification is that it will last longer to fade away. You might need this if there's a lot of text to be read, too much text could be considered bad design, though. The correct way to respond to a notification is to just open the associated application/status icon.

Read on Notifications, indicators and alerts to see Mark's thinking when the concept was considered. Also, you might find useful to read How to customize on screen notifications?

No, you can't. That notification is just a notification and you cannot do anything else with it. And also, unfortunately, cannot be configured somehow. The single purpose that I can see is that it provides you a... notification.