[SPAM] in subject headers

[Shane]

Running outlook 2000 through a fairly default exchange
2000 box. Since last thursday, when i loaded SP4 to the
box, all email from external sources have come through
without any alteration to the subject line. Now, many
external emails come with a [SPAM] tag in the subject
line. All internal mail is fine. The clients had no
changes made to it.

Is it just coincidence or has the service pack changed
something somewhere? Could possibly the ISP be doing this?

Thanks
Shane