Introduction
If your server is unable to receive email, it could be the result of a firewall blocking port 25 inbound. Port 25 inbound must be open for your cPanel server to receive email.
Procedure
You can use the nmap tool to check whether port 25 inbound is blocked. Nmap would need to be run from outside the server to test for this. For example:
nmap $ipaddress -p 25
*You would replace $ipaddress with the public IP address of your server.
If the state shows 'filtered,' it would indicate a firewall is blocking inbound port 25.