How frequently should Outlook check for new mail on the server?

For optimal service in both Outlook and on the mail server, no less than 8 -10 min, longer if you use a dialup account. Every min or two is too frequent!

Why? Three reasons:

1. Outlook doesn't mark mail as downloaded until after it finishes downloading all mail. If Outlook starts a new collection before it's finished with the first one you'll either end up with duplicates or Outlook will lock up and you'll need to reboot the computer.
2. Be kind to your ISP. Checking for new mail uses resources on the server. If every account holder would check for new mail every minute, the ISP would need new servers to handle the load, or fewer subscribers.
3. Don't hog the connection - leave it free so other subscribers can use it. If everyone tries to check mail at once, some users will experience timeouts. This is because there are a limited number of connections to the mail server. By using a higher setting, the chances of everyone checking at once is lowered and everyone has a better experience.

If you feel your email is so important that you or your business would die with checks less often than 8 minutes, then email isn't the right medium for you. Use instant messaging or the telephone for urgent matters that can't wait 10 minutes.