If you get a message like the one following, then you are filtering emails through spamassasin using /usr/bin/spamc but the spamassassin service is not running.
Oct 24 12:27:31 vps131 postfix/smtpd[32273]: connect from mail.example.com[10.1.1.25]
Oct 24 12:27:31 vps131 postfix/smtpd[32273]: 4EDA01DB08012: client=mail.example.com[10.1.1.25]
Oct 24 12:27:31 vps131 postfix/cleanup[32280]: 4EDA01DB08012:
message-id=<200710241227.29534.chris@electrictoolbox.com>
Oct 24 12:27:31 vps131 postfix/qmgr[16073]: 4EDA01DB08012: from=<chris@electrictoolbox.com>, size=1161, nrcpt=1 (queue active)
Oct 24 12:27:31 vps131 postfix/smtpd[32273]: disconnect from mail.example.com[10.1.1.25]
Oct 24 12:27:31 vps131 spamc[3173]: connect(AF_INET) to spamd at 127.0.0.1 failed, retrying (#1 of 3): Connection refused
Oct 24 12:27:32 vps131 spamc[3173]: connect(AF_INET) to spamd at 127.0.0.1 failed, retrying (#2 of 3): Connection refused
Oct 24 12:27:33 vps131 spamc[3173]: connect(AF_INET) to spamd at 127.0.0.1 failed, retrying (#3 of 3): Connection refused
Oct 24 12:27:34 vps131 spamc[3173]: connection attempt to spamd aborted after 3 retries
Oct 24 12:27:34 vps131 postfix/pipe[32282]: 4EDA01DB08012: to=<hhh@mail.example.com>, orig_to=<test@example.com>, relay=maildrop, delay=3.1, delays=0.03/0/0/3, dsn=2.0.0, status=sent (delivered via maildrop service)
Oct 24 12:27:34 vps131 postfix/qmgr[16073]: 4EDA01DB08012: removed
So you need to start it:
/etc/init.d/spamassassin start
And set it so it starts automatically when the server starts (example below is for Red Hat Enterprise Linux, CentOS and Fedora based Linux distros):
chkconfig spamassassin on