We have mail server from plesk and spamassassin service also is running. As we wanted to prevent spam, I have added below entries in ~/.spamassassin/user_prefs file,
rewrite_header Subject *****SPAM*****
now it is detecting the spams properly and rewriting the header subject as spam. Now I don't wanted those spams to be in users inbox instead we would like to move those spam emails to spam folders automaticaly. Since I am fresher in this, expecting your kind help to achieve the same.