Deploying an Anti-Spam Filter for Your VPS Mail Server

Spam emails can flood your mail server, consume resources, and damage your domain reputation. Attackers use spam to distribute malware, phishing attempts, and fraudulent messages. Deploying an anti-spam filter ensures that only legitimate emails reach your inbox.

Install SpamAssassin

SpamAssassin is a widely used open-source spam filter that scans emails for spam characteristics.

sudo apt update
sudo apt install spamassassin
sudo systemctl enable spamassassin
sudo systemctl start spamassassin

Enable SpamAssassin for Postfix:

sudo nano /etc/postfix/master.cf

Add the following:

spamassassin unix - n n - - pipe
  user=spamd argv=/usr/bin/spamc -f -e /usr/sbin/sendmail -oi -f ${sender} -- ${recipient}

Restart Postfix:

sudo systemctl restart postfix

Configure SpamAssassin Rules

Edit the configuration file to adjust spam detection thresholds:

sudo nano /etc/spamassassin/local.cf

Example settings:

required_score 5.0
rewrite_header Subject ***SPAM***
use_bayes 1
bayes_auto_learn 1

Restart SpamAssassin:

sudo systemctl restart spamassassin

Enable Postscreen in Postfix

Postscreen helps block spam bots before they connect. Edit the Postfix configuration file:

sudo nano /etc/postfix/main.cf

add:

postscreen_enable = yes
postscreen_greet_action = enforce
postscreen_dnsbl_sites = zen.spamhaus.org bl.spamcop.net b.barracudacentral.org

Restart Postfix:

sudo systemctl restart postfix

Use Rspamd for Advanced Filtering

Rspamd is a modern spam filtering system with better performance than SpamAssassin.

sudo apt install rspamd

Enable integration with Postfix:

sudo nano /etc/postfix/master.cf

Add:

rspamd unix - n n - - pipe
  user=_rspamd argv=/usr/bin/rspamc -f -e /usr/sbin/sendmail -oi -f ${sender} -- ${recipient}

Restart Postfix:

sudo systemctl restart postfix

Enable Greylisting with Postgrey

Postgrey temporarily rejects unknown senders to filter out spam bots.

sudo apt install postgrey
sudo systemctl enable postgrey
sudo systemctl start postgrey

Edit Postfix configuration:

sudo nano /etc/postfix/main.cf

add:

smtpd_recipient_restrictions = permit_mynetworks,
   permit_sasl_authenticated,
   reject_unauth_destination,
   check_policy_service inet:127.0.0.1:10023

Restart Postfix:

sudo systemctl restart postfix

Use DNS Blacklists (DNSBLs)

Blocking known spam sources using DNSBLs improves email filtering. Edit Postfix configuration:

sudo nano /etc/postfix/main.cf

add:

smtpd_recipient_restrictions = reject_rbl_client zen.spamhaus.org,
   reject_rbl_client bl.spamcop.net,
   reject_rbl_client b.barracudacentral.org

Restart Postfix:

sudo systemctl restart postfix

Monitor and Adjust Filters

Regularly check logs for spam detection performance:

tail -f /var/log/mail.log
tail -f /var/log/syslog

Adjust filter thresholds and blocklists based on spam trends.