Check imap_ns_* settings. Also enable imap_debug and show the log.
in config
$rcmail_config['imap_ns_personal'] = null;
$rcmail_config['imap_ns_other'] = null;
$rcmail_config['imap_ns_shared'] = null;
imap_log
[20-Dec-2010 14:26:33 +0300]: S: * OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION] Courier-IMAP ready. Copyright 1998-2008 Double Precision, Inc. See COPYING for distribution information.
[20-Dec-2010 14:26:33 +0300]: C: A0001 CAPABILITY
[20-Dec-2010 14:26:33 +0300]: S: * CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION
[20-Dec-2010 14:26:33 +0300]: S: A0001 OK CAPABILITY completed
[20-Dec-2010 14:26:33 +0300]: C: A0002 AUTHENTICATE PLAIN
[20-Dec-2010 14:26:33 +0300]: S: +
[20-Dec-2010 14:26:33 +0300]: C: Mzc0NzE3NDM4MTIxQHdta2VlcGVyLmNvbQAzNzQ3MTc0MzgxMjFAd21rZWVwZXIuY29tAE0yMGdZV2RR
[20-Dec-2010 14:26:33 +0300]: S: A0002 OK LOGIN Ok.
[20-Dec-2010 14:26:33 +0300]: C: A0003 LSUB "" "*"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.INBOX.Redmine"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.&BB8EPgQ0BDQENQRABDYEOgQw- &BD8EPgRHBEIEMA-"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.&BB8EPgQ0BDQENQRABDYEOgQw-"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.&BBAEQARFBDgEMg-"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\Marked \HasChildren) "." "INBOX"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Trash"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Sent"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Drafts"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Doubt"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\Noselect \HasChildren) "." "INBOX.INBOX"
[20-Dec-2010 14:26:33 +0300]: S: A0003 OK LSUB completed
[20-Dec-2010 14:26:33 +0300]: C: A0004 GETQUOTAROOT INBOX
[20-Dec-2010 14:26:33 +0300]: S: * QUOTAROOT "INBOX" "ROOT"
[20-Dec-2010 14:26:33 +0300]: S: * QUOTA "ROOT" (STORAGE 40820 488282)
[20-Dec-2010 14:26:33 +0300]: S: A0004 OK GETQUOTAROOT Ok.
[20-Dec-2010 14:26:33 +0300]: C: A0005 LOGOUT
[20-Dec-2010 14:26:33 +0300]: S: * BYE Courier-IMAP server shutting down
[20-Dec-2010 14:26:33 +0300]: S: A0005 OK LOGOUT completed
[20-Dec-2010 14:26:33 +0300]: S: * OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION] Courier-IMAP ready. Copyright 1998-2008 Double Precision, Inc. See COPYING for distribution information.
[20-Dec-2010 14:26:33 +0300]: C: A0001 CAPABILITY
[20-Dec-2010 14:26:33 +0300]: S: * CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION
[20-Dec-2010 14:26:33 +0300]: S: A0001 OK CAPABILITY completed
[20-Dec-2010 14:26:33 +0300]: C: A0002 AUTHENTICATE PLAIN
[20-Dec-2010 14:26:33 +0300]: S: +
[20-Dec-2010 14:26:33 +0300]: C: Mzc0NzE3NDM4MTIxQHdta2VlcGVyLmNvbQAzNzQ3MTc0MzgxMjFAd21rZWVwZXIuY29tAE0yMGdZV2RR
[20-Dec-2010 14:26:33 +0300]: S: * OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION] Courier-IMAP ready. Copyright 1998-2008 Double Precision, Inc. See COPYING for distribution information.
[20-Dec-2010 14:26:33 +0300]: C: A0001 CAPABILITY
[20-Dec-2010 14:26:33 +0300]: S: A0002 OK LOGIN Ok.
[20-Dec-2010 14:26:33 +0300]: C: A0003 STATUS INBOX.Redmine (MESSAGES UNSEEN)
[20-Dec-2010 14:26:33 +0300]: S: * CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA IDLE AUTH=PLAIN ACL ACL2=UNION
[20-Dec-2010 14:26:33 +0300]: S: A0001 OK CAPABILITY completed
[20-Dec-2010 14:26:33 +0300]: C: A0002 AUTHENTICATE PLAIN
[20-Dec-2010 14:26:33 +0300]: S: A0003 NO Mailbox does not exist, or must be subscribed to.
[20-Dec-2010 14:26:33 +0300]: S: +
[20-Dec-2010 14:26:33 +0300]: C: Mzc0NzE3NDM4MTIxQHdta2VlcGVyLmNvbQAzNzQ3MTc0MzgxMjFAd21rZWVwZXIuY29tAE0yMGdZV2RR
[20-Dec-2010 14:26:33 +0300]: C: A0004 LOGOUT
[20-Dec-2010 14:26:33 +0300]: S: * BYE Courier-IMAP server shutting down
[20-Dec-2010 14:26:33 +0300]: S: A0004 OK LOGOUT completed
[20-Dec-2010 14:26:33 +0300]: S: A0002 OK LOGIN Ok.
[20-Dec-2010 14:26:33 +0300]: C: A0003 LSUB "" "*"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.INBOX.Redmine"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.&BB8EPgQ0BDQENQRABDYEOgQw- &BD8EPgRHBEIEMA-"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.&BB8EPgQ0BDQENQRABDYEOgQw-"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.&BBAEQARFBDgEMg-"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\Marked \HasChildren) "." "INBOX"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Trash"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Sent"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Drafts"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\HasNoChildren) "." "INBOX.Doubt"
[20-Dec-2010 14:26:33 +0300]: S: * LSUB (\Noselect \HasChildren) "." "INBOX.INBOX"
[20-Dec-2010 14:26:33 +0300]: S: A0003 OK LSUB completed
[20-Dec-2010 14:26:33 +0300]: C: A0004 LOGOUT
[20-Dec-2010 14:26:33 +0300]: S: * BYE Courier-IMAP server shutting down
[20-Dec-2010 14:26:33 +0300]: S: A0004 OK LOGOUT completed