How to check SMTP_BLOCK option in server firewall on CentOS Linux System ?

SMTP_BLOCK option help to prevent spam email abuse on a CentOS Linux server that bypasses exim and sends spam emails out through port 25. By enabling this option on server you can bind the spammers to send out spam emails using socket connection from your server using any web scripts as it will use the exim or sendmail binary instead. It will block all SMTP excluding root, exim and mailman (forces scripts/users to use the exim/sendmail binary instead of sockets access). This replaces the protection as WHM > Tweak Settings > SMTP Tweaks. This option uses the iptables ipt_owner module and must be loaded for it to work. It may not be available on some VPS platforms.

You can check whether this option is enabled or not through WHM or Command line, and enable or disable it as per your need.

Through WHM :
Step 1). Login to WHM as root.

Step 2). Check for "Firewall Configuration" csf.config file under "ConfigServer Security & Firewall" category.

Step 3). Now open this file and look for "SMTP Settings" tab and check value for "SMTP_BLOCK". By default it is 0 which means off or disabled.
----
SMTP_BLOCK=0
----
You can enable it by changing it's value to 1.
----
SMTP_BLOCK=1
----

If SMTP_BLOCK is enabled but you want to allow local connections to port 25 on the server (e.g. for webmail or web scripts) then enable "
SMTP_ALLOWLOCAL" option to allow outgoing SMTP connections to the loopback device.
----
SMTP_ALLOWLOCAL=1
----

Click to expand...

Step 3). Now save file and restart CSF (Server Firewall) to apply changes.