Outlook takes focus when I get mail - can I make it stop?

I usually keep Outlook in the background when I'm working away. Then whenever I receive a new email message, Outlook will take the "focus" ... in other words come to the front. Is there a way I can stop this behavior? It is getting very distracting.

I'll be working in Photoshop or InDesign, and then juuust as I'm about to click on an item ... BOOM... Outlook fills up the screen and I find myself clicking some random mail message instead.

I know Outlook is eager to tell me I have new mail. But I'd rather check it when it's convenient for me.

I'm not familar with the current Outlook, but will it do this if you "hide" the program? Mail doesn't.

I got a solution! It looks like it was Spaces that was causing the issue. I had turned on Spaces a long time ago thinking I might find it useful, but I never got around to trying it out. As soon as I turned it off, my Outlook started behaving. Go figure.

And just to add ... yes, the annoying behavior would still happen even if I had Outlook hidden or minimized. Very odd.