Many providers now block outbound connections on port 25, which means that a cPanel system would need to configure a smarthost to allow for outbound emails to function. This resource covers how to setup a smarthost using a second cPanel server on a different network.
In the following instructions "Webserver" refers to the system that is on a network that blocks port 25 while "Mailserver" refers to a second system that will be used to relay email through.
On the Webserver configure the following:
On the Mailserver configure the following:
Trusted SMTP IP addresses -
Backup MX hosts -