After deleting emails, the disk usage page in cPanel shows "Other Usage".
cPanel has two mail storage types.
- Mdbox ( Optimized for performance to reduce the number of files in use by a mail account )
- Maildir ( Optimized for performance and can use the same file to deliver messages to multiple users )
See the documentation for more information.
WHM - Email - Mailbox Conversion
When Mdbox is in use, the emails aren't deleted from the filesystem until cPanel updates run the dovecot-maintenance script. So until this task is completed, deleted emails from an account will show as using "other" usage.
Run the maintenance script manually to clear out the deleted emails immediately.