If I were designing it, I would have separate new mail pop-ups for each account. If you click the alert and have 1 new message, it should open that message. If you have more than 1 new message, it should take you to the message list of your inbox, scrolled to where the oldest new message is visible, at the bottom of the screen if there are more than 1 screen's worth of new messages.

It is a matter of opinion. It rings once, I click the banner, it takes me to read the email. Then read another, assume I'm done, move on. Little do I know there's another in some other account, for example.

In other phones, the little status area @ keeps you posted. On N900, if you missed it, you missed it. And frankly I don't want to hunt the last unread e-mail, especially with GMail that throws it in some archived folder somewhere. Sometimes it's not even on top of the pile.

Also, I see it takes you to the deepest level it can where you can overview the changes. Don't know how often you get email, but I couldn't possibly always know via > and < whether this is the first time I see the email. I'd probably read newest and previous to newest in inbox 1 then exit and miss the second email in inbox 2.

However, as I've agreed, it is a matter of opinion. Me, personally, if I have 10 new emails I have to hunt through 15 folders in whatever order they are I'd lose my count. But that's just me.