leaving mail on the isp

I am running since short postfix on my 8.1 in order to get my email from my ISP via a dial-up. Before that I got my mail via Kmail and in most cases left my mail on the isp in order to retrieve it also from another place. Since running postfix email is downloaded on my computer but is also removed from my isp account. I have looked into the available info to see if I could leave email on the isp but did not find anything. Could somebody help me? -- NTReader v0.36w(P)/Beta (Registered) in conjunction with Net-Tamer.