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Am trying to setup Mandrill for transactional emails for one of my web application. I decided to use the SMTP method(credentials from Mandrill) than using API as it would be easy for me.

In my VPS server, SMTP Restriction is enabled. Screenshot: from WHM panel

So, when I send email, it redirects back to the local server as the outgoing SMTP is allowed only for root, exim and mailman (SMTP Restriction).

If I disable the SMTP Restriction, the email would be sent successfully! When I checked whether there's any feature to whitelist this Mandrill SMTP server only, while keeping the SMTP Restriction, I came to know that the cPanel/WHM doesn't have that feature.

Upon further research, I found that this whitelisting can be done via CSF Firewall, which I have already installed in the server.

So, when I checked CSF > Firewall Configuration > SMTP Settings > SMTP_BLOCK, it doesn't show anything where I can "whitelist" a particular outgoing SMTP server. Below that option, this is mentioned:

This option uses the iptables ipt_owner/xt_owner module and must be loaded for it to work. It may not be available on some VPS platforms

Unfortunately, when I contacted my hosting provider, they aren't giving me exact information, instead giving me vague answers. So that's why I decided to ask here.

So, my question is, how to add the Mandrill SMTP server to the whitelist while keeping the SMTP Restriction enabled!

Thank you

EDIT

Upon further research, I got this info: https://community.centminmod.com/threads/csf-firewall-info.25/#post-6613

When I added nginx to the SMTP_ALLOWGROUP, it didn't worked. My server is CentOS btw.

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