Postfix Spam, how do I debug?

There are two common causes:

1. Open Relay
2. Bad security on a website <form>

Open Relay Open relay just means that anyone can use your email server to send out emails and pass them along. Obviously this is very bad for Spam, a simple way to disable it is to setup postfix to only accept sending email from localhost which is explained here: https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-as-a-send-only-smtp-server-on-ubuntu-16-04#step-2-—-configuring-postfix

Bad Security The second biggest cause is that you have a webform somewhere that sends out emails and that it wasn’t properly secured or the code itself has a few security issues in which case people can abuse that web form to send out emails, that takes a bit more work to troubleshoot but if you check your webserver logs you can see if there are a large number of requests to a contact form that exceed what your expected normal usage is.

Heya,

You can start by examining the Postfix logs to identify the source of the spam:

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

Look for patterns such as specific scripts, users, or IP addresses that are sending a high volume of emails.

You can use a tools like maldet, rkhunter or chkrootkit to scan for malware. All tools will return a list of pottentially compromised files which you can check and then remove if necessary.

Also to prevent unauthorised use of Postfix, configure it to only allow sending emails from authenticated users and authorized IP addresses:

You can edit /etc/postfix/main.cf to enforce stricter access policies:

smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination 
smtpd_sender_restrictions = reject_unknown_sender_domain, reject_unverified_sender 
smtpd_client_restrictions = permit_mynetworks, reject_rbl_client zen.spamhaus.org, reject_rbl_client bl.spamcop.net 
smtpd_sasl_auth_enable = yes 
smtpd_sasl_security_options = noanonymous

A good approach will be to set up monitoring to detect unusual server activity in real-time. Tools like fail2ban can help in blocking malicious IPs.

Hope that this helps!