Gmail – What Do These Icons Indicate?

You can simply do a search for one of the following:

@googlecode.com

from:(*@googlcode.com)

Or you can create a filter to automatically label incoming messages from @googlecode.com. You can also apply the filter retroactively by selecting Also apply filter to X matching conversations.

I suggest using a filter and label, as you can choose different ways to handle matching messages. For example, if you want to reduce clutter, you can have messages Skip the inbox.

The official priority inbox video explains very clearly how it works.

Basically you need to teach to your Gmail how to recognise an important email to obtain a descent result.