Evolution "Check custom headers for junk" not working

[chmac]

My server run spamassassin on incoming mail. Evolution has an option under Edit > Preferences > Mail Preferences > Junk called "Check custom headers for junk". The default options are:
X-Spam-Flag = YES
X-Spam-Level = ****

I've selected the option. I've selected "Check incoming messages for junk".

When a mail arrives with those two header set, it doesn't get put into the junk folder. It appears as normal in my regular folder. Anyone know how I can change this?

I had it set as a filter, but processing filters can be quite slow when I download a lot of mail (over IMAP).

Any suggestions would be appreciated.