Mail Delay

We have a Mail Server Running Microsoft Exchange 2000, our computers are primarily running WindowsXP Pro. The problem is that there seems to be a problem with everyone sending and receiving mail, locally and not. When inside of Outlook XP and 2000 I have everyone's workstation set to send and receive automatically every 1 minute, yet somehow when someone sends an email to another within the office the mail is not delivered or received for nearly 20-30 minutes. The only way to do so is to look at the outbox, at which point it sends. Finally the recipient must click on an old message in the inbox for the new message to show up. I've checked every possible setting I can think of with no avail.