How are the files encrypted?

I'm taking over a domain. Inside Cpanel > Mail directory, I have files of multiple users, For one user, the email file seems to be encrypted when I attempted to open it using Notepad. How do I configure such that the other users also have their emails stored encrypted inside this Mail directory? Also, I see some files have "RSZ", "SabZ", "RsabZ" at the end. What do they mean? Is there a place where I can read up more on the file naming convention for the email files stored in this directory? Thanks!