[CentOS] Junk mail filtering in Evolution

I'm upgrading a few clients to CentOS 4.2.  They have been using
Evolution 1.x, so this is the first release that will have junk mail
filtering.  So I've switched from Thunderbird to Evolution on my own
desktop tp better support them.

I have been dutifully marking the spam and the ham.  But the filter has
so far not flagged a single email on its own. (I've flagged about a
hundred so far, so it's had time to learn.)

Is there something in the config that I need to set to get it to filter?

One possible kink is that my own mail account uses imap.  My users will
be using pop3.  

Thanks,
Steve