Why does my email user have both the "cur" and "mailboxes" folders?
When using the Mailbox Conversion WHM feature or the script, if you don't choose "-delete-old-format", the folders in the old format will remain in the user's account, so now the user is using twice as much disk space in the mail folder. The contents of this article are replicated from our Mailbox Conversion documentation.
Solution
The mdbox format
To create disk space after you convert your mailboxes to the mdbox format, delete the following folders:
cur
.Drafts
.Junk
new
.Sent
tmp
.Trash
The maildir format
To create disk space after you convert your mailboxes to the maildir format, delete the /mailboxes directory.