It very much depends on how you have your server configured; there are perhaps a half-dozen common ways of setting up the two - qmail-scanner, .qmail-default files, qmail-queue replacements... which way are you trying to do it? Not to belabor the obvious, but have you made sure Spamassassin is configured as you wish, and actually running? Is it adding headers to incoming mail?
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the server has plesk but i wish to use spamassassin to stop receiving mails with spam.
I use at spamassassin rbl too but it dosen't rewrite subject on spam mails and i wish to stop spammers usin spamassassin dcc razor and pyzor.
i can't install qmail scanner because i can't find qmail queue patch it's required