Of course, Mail has some settings built-in, via a popup menu in the Mail Preferences. But what if checking for new mail every minute is too fast, and every 5 minutes is too slow?
Well, it’s just a plist entry: ~/Library/Preferences/com.apple.mail.plist
The entry you’re looking for is “Poll Time” and you enter the number of minutes.
The issue with this is that you have to do it every time you change the preferences, since it will revert to one of the choices in the popup menu.