cPanel currently only supports Maildir. Maildir works with Courier.
Maildir is a directory that stores email messages as files.
The Courier mail server provides folders and quotas for the email accounts.
The folders inside your mail directory are additional subdirectories such as .Drafts .Trash and .Sent. Each folder contains three subdirectories called tmp, new, and cur.
tmp
The 'tmp' subdirectory temporarily stores email messages that are in the process of being delivered. (It may also store other kinds of temporary files.)
new
The 'new' subdirectory stores messages that have been delivered, but have not yet been seen by any mail application, such as webmail or Outlook.
cur
The 'cur' subdirectory stores messages that have already been viewed by mail applications, like webmail or Outlook.