Configure server-wide Spamassassin

pico /etc/mail/spamassassin/local.cf
Add this:

report_safe 0
required_score 12

# The default score for BAYES and SURBL is too low compare to my reject threshold(see section5) at 15
score BAYES_99 6.00

# Check here for the detail of the URIBL data sources
# http://answers.google.com/answers/threadview?id=422251
score URIBL_SBL 4.50
score URIBL_AB_SURBL 6.50
score URIBL_OB_SURBL 4.50
score URIBL_SC_SURBL 6.00
score URIBL_WS_SURBL 4.50
score URIBL_PH_SURBL 6.00


# Enable server-wide Vipul's Razor in Spamassassin
# Remove razor_config line if you want to disable Vipul's Razor
razor_config /var/spool/mqueue/.razor/razor-agent.conf


# Enable server-wide DCC in Spamassassin
# Remove dcc_home line if you want to disable DCC
dcc_home /var/dcc

5. Spam Protection
Exiscan allow exim invoking SpamAssassin to obtain $spam_score and $spam_report. If you install all programs in the section1, scored will calculate from Spamassassin+SARE+Razor+DCC. Since the false positive rate with spam scanning is high compared to virus scanning, it is wise to implement a scheme with multiple spam thresholds with different actions. As my preferences, I set score at 15 for reject thresholds, 12

You could reduce the reject theshold from 15 to 11 or whatever you want. But it increase the chance you reject the valid email.

Another way is increase the score for specific SA rules such as SURBL rules.

score URIBL_SBL 4.50
score URIBL_AB_SURBL 6.50
score URIBL_OB_SURBL 4.50
score URIBL_SC_SURBL 6.00
score URIBL_WS_SURBL 4.50
score URIBL_PH_SURBL 6.00

You may increase it. But again it increase the chance you reject the valid email.

No. As email senders are always fake. You server will be blocked from variuos RBL, and big guy such as AOL, Hotmail, and etc.

Valid email doesn't mean it is not faked. It will worsen if your auto email sent to innocent people. That means you are spamming to prevent spam mail. It doesn't right logic.