Lost emails from Mail (on intel iMac)

[Gaz05]

I have a POP account with my ISP and I understood that when I read emails a copy was downloaded from the ISP server to my iMac. However, the ISP has recently started deleting emails off their server after 45 days and when that happens I can no longer see those emails on my computer (even though I have downloaded a copy previously). I get an error message saying that I have to download the email again. It must be caused by a setting on my iMac because this problem affects my account but not that of my wife. I can't find a setting on the iMac to change to fix the problem. Does anyone have any suggestions how to fix this problem as I need to keep copies of certain emails for long periods of time. Thanks.