Delays in some outgoing email

I a running Exchange 2000 with Outlook 2000 clients. We use an internet-based SMTP server. Periodically, an email will not reach it’s recipient for 8-24 hours. I can see that it arrives at our SMTP provider immediately, but can’t seem to resolve the reason for the delay. The vast majority of the mail gets delivered immediately. Is there a way to determine whether the problem is with our SMTP server or further downstream? Why is mail to certain locations being delayed?