How do I secure my SMTP email with encryption?
If you're referring to a PCI vendor scan and the report says your SMTP server allows Cleartext Login Permitted for SMTP (Simple Mail Transfer Protocol), then here's the solution.
Navigate to WHM's Exim Configuration Manager interface
(Home >> Service Configuration >> Exim Configuration Manager).
Enable the Require clients to connect with SSL or issue the STARTTLS command before they are allowed to authenticate with the server option.
This should address the issue with your PCI vendor.