if you are going to be downloading email from your ISP you need to fiddle with fetchmail to put them in the right users mailbox.
if your going to host multiple users its easyer to have your mail delivered right to your own mailserver rather then your ISP one. however since at the moment you are lacking experience with your current mail setup id suggest trying this with a test domain first. test.kleinakrootoyota.co.za for example. and setup some mailboxes under that.