You've Got Mail (Company) can help you stop your heavy e-mail program to be running in the background all the time, but still can the instant e-mail notification that for example Messenger provide.

It features that:
Check for mail every X seconds.
Multiple e-mail accounts
Play sound
Show popup balloon
Show Messagebox
Flash systray icon
Run e-mail program
Send e-mail from systray
Check e-mail from systray
Run e-mail program from systray