There can be several reasons for this. One of which is that you have received an e-mail message with a large attachment. An example would be that someone sent you an e-mail with a 10MB attachment. If you connect on a 28.8 Kbps modem, this would take approximately 47 minutes to download alone!
Another possibility is that there are connectivity problems between your ISP and ZA Web Hosting, causing your e-mail to download slower than normal.