Below instruction is for only changing one cPanel account's outgoing EXIM IP address.
Note: This requires a dedicated IP for that specific domain
1. Log into WHM as root
2. Access into Service Configuration
3. Select Exim Configuration Manager
4. Select Domains and IPs tab
5. Turn option to "ON" for Send mail from the account's IP address
For more detailed step on how you will be able to configure custom EXIM IP, please review the following documentation: