My ISP is Yahoo!, which uses its own antispam program. My operation is to download all messages to my computer, rather than keeping them on line.
Their program learns from usage, I'm told. As soon as my current glob of messages is downloaded, the program identifies and marks what it considers spam, and moves it to a JUNK folder, where it will stay for a period of time, and then be automatically moved to TRASH.
Because I subscribe to four mail lists, I get anywhere from 150 to 200 emails every day. Of that, perhaps as many as 50 are legitimate. Of the 150 or so spams, the program marks and moves maybe 135 to JUNK. I then scan through the list and declare the remaining spams as spam, and they are automatically moved to JUNK. Every day I take a quick look in the JUNK folder list, just so I can un-spam any legitimate messages, and then I move those back to my IN-BOX. There are VERY few of those.
So I have a number of false negatives, but almost no false positives. At one time I also ran Norton's spam eliminator, but I found that it moved too many legitimate messages, so I cut back, to rely only on the Yahoo! system.
Because old spam in the JUNK folder are moved to TRASH automatically, I do a quick run-through my TRASH list every day, to delete spam, all of which are easy to see becase (A)They are marked, and (B) they are in bold, because unread. Just a matter of setting the filter to show only unread messages and holding down the delete button for a while.