Clean email Default Account without loosing emails

Hello As some other users in threads I read, we have this issue and tried to clean the default email account from messages and folders that seem a copy of the email accounts created. I read they are copies but, when we deleted some, the users noticed the emails gone from their inboxes too. What do I need to do to fix this... some default accounts are way to big, gigabytes big. Other questions regarding this issue: 1) Default email set to discard the email while your server processes it by SMTP time with an error messages. Can I use discard all safety although cPanel says "not recommended? 2) Use the cPanel's free space functionality safely and not loose other accounts emails 3) For space, functionality and security (SSL and spam protection), what is better to use (set at WHM) mdbox or maildir without issues with the different email clients? 4) Any recommended settings in the Exim Configuration Manager? 5) Any other recommendations to improve email performance and security? Thanks a lot! aB