send/receive emails

A strange problem with outlook express. Sending emails works just fine, all settings have been checked and rechecked, ISP tech support has said all settings are correct. However, although there is no problem sending emails, receiving emails is a problem. No matter who the sender, or what ISP. The problem is that unless replying to an email which was sent, no emails can be received. Any ideas?

System is running Windows 2000 Pro with SP3