That's why ISPs rely on their customers marking emails as spam.
By doing so you are helping to "educate" your ISP's spam filters. In the main, spam filters work by recognising the IP address or email address of the sender - not the title of the email. Eventually the filters should catch up and recognise spam as such.
My contacts at BT say that they have noticed a large increase in spam being generated recently.
Are PCs still using Windows XP coming back to bite us?????