Block direct email delivery

[peopleinside]

Hi, in this day my server is under spam attack, for this reason i have activated a Gateway guard solution to my MX Records.
Some spam maybe are able to bypass MX record so continue to arrive on my inbox.

I need ask to the CSF Firewall to deny all IP except some IP who will be this MX Guard IP.
How i can do this?

[scrupul0us]

Block port 25 in csf.conf and then in csf.allow:

tcp|in|d=25|s=change-to-mx-guard-ip

...add as many lines as you have IP's to trust then reload CSF:

csf -r

HTH