exchange server – internet e-mail

I have an Exchange server 5.5 set up in Windows NT. The internal mail deliverage works fine, and sending external mail over the internet works fine and fast as well. the problem however is receiving internet mail on my server. when other people sendmail to an adress on my exchange server it can take up to a day before it arives. the header on the e-mails says the mail arives at my ISP one minute after it’s sent, and I have set up MY server to check and send the mail every 20 minutes on the ISPover an ISDN modem, but it just doesn’t receive any external mail untill 7 am the next morning apparently. How can this be ?