To send outgoing email from all cPanel accounts on a dedicated IP address, you would need to enable the "Send mail from the accounts's IP address" option in the Exim Configuration Manager.
1. Navigate to WHM: Home -> Service Configuration -> Exim Configuration Manager
2. Click on the "Domains and IP" Tab
3. Enable the "Send mail from accounts's dedicated IP address"
4. Click 'Save' at the bottom
The above steps will affect all accounts using dedicated IPs on the server.
Sending email from a dedicated IP address means that you will need to modify the PTR record for the dedicated IP address to match the account's domain name.
You may need to reach out to your Hosting Provider if your DNS Server does not have the authority to modify the PTR record. Otherwise, you can create the PTR (rDNS) using the following documentation on the cPanel server:
If you would like more information, please review this link.