Do the following to get rid of the errors

as well as the INBOX.TRASH, INBOX.SENT,

INBOX.DRAFTS listed in the left panel

$default_folder_prefix          = 'INBOX.';
$trash_folder                   = 'Trash';
$sent_folder                    = 'Sent';
$draft_folder                   = 'Drafts';
THEN HERE IS WHAT WAS MISSED... by default, the delimiter

in IMAP Settings was "/" when it should be "."

Should look like:

IMAP Settings
--------------
4. IMAP Server : localhost
5. IMAP Port : 143
6. Authentication type : login
7. Secure IMAP (TLS) : false
8. Server software : courier
9. Delimiter : .
In doing so, the left side folder view in SquirrelMail

will look like the following:

INBOX
  Drafts
  Sent
  Trash
Rather than:

INBOX
INBOX.DRAFTS
INBOX.SENT
INBOX.TRASH
Which looked silly!

I'm using the Mailutils-Imapd4 and had to use ./ under Default Folder

// Terese

Please Select none under Default folder

To do this so it's correct for all your users cd config/ and run ./conf.pl select option 3 then 1 and enter 'none'. You don't need the need the quotes. When the menu shows again make sure the entry for 1 is blank. Save your changes and exit.