Lengthen Outlook Mail Notifications

I don't know of any way to make the new-mail pop-up halt the screen saver. Your best bet is to set the Desktop Alert to stay visible long enough that it will still be there when you return to the computer. Outlook limits the alert duration to 30 seconds, but a Registry tweak can set it for much longer. 

Launch Regedit from the Start menu's Run dialog and navigate to the key :

HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Common\DesktopAlert Look in the right-hand pane for a value named TimeOn. It probably won't be there; in that case, right-click in the right-hand pane, choose New | DWORD value, and name the value TimeOn. Now double-click it and click the Decimal option. Type in a value equal to the number of milliseconds you want the alert box to remain visible. There are 86,400,000 milliseconds in a day, so that's a good value to use. Click OK and close Regedit. You may have to restart Outlook before it will notice the change. But now the desktop alerts will remain visible for a whole day (or until you click the little X icon at top right).