There's an option in preferences to 'Group documents from each application on a tab'. So all emails you have open end up on a single 'Mail' tab in the Notes client. It's a handy poweruser type feature.

However, navigation would be simpler (particularly if you only have 2 or 3 open from an app) if repeatedly clicking on the tab cycled through the various documents so you don't HAVE to use the drop-down to find the one you want.

I don't know if it's me, but the one that Notes puts at the 'front' is invariably not the one I'm looking for.