Postfix mail problem in ubuntu 8.04

I am loggen in usermin and trying to access mail inbox getting below error message:
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An error occurred listing mail in this folder : Failed to login to POP3 server : Internal error occurred. Refer to server log for more information. [2008-06-12 13:13:26]

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When sending mail via Usermin there is no problem. I compose and sent mail to gmail and hotmail.

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log from /var/log/mail.log ( No Prblem sending mail )
Jun 12 13:12:36 nandini postfix/pickup[5639]: 671EF9E09D: uid=1002 from=
Jun 12 13:12:36 nandini postfix/cleanup[9400]: 671EF9E09D: message-id=<1213272756.9426@kailashvision.net>
Jun 12 13:12:36 nandini postfix/qmgr[1145]: 671EF9E09D: from=, size=673, nrcpt=1 (queue active)
Jun 12 13:12:36 nandini dovecot: IMAP(admin.kailashvision): Connection closed
Jun 12 13:12:37 nandini postfix/smtp[9402]: 671EF9E09D: to=, relay=mx2.hotmail.com[65.54.244.40]:25, delay=1.1, delays=0.02/0/0.53/0.58, dsn=2.0.0, status=sent (250 <1213272756.9426@kailashvision.net> Queued mail for delivery)
Jun 12 13:12:37 nandini postfix/qmgr[1145]: 671EF9E09D: removed

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Problem with all user:
New user added as admin@kailashvision.net but same error

when trying to acces mail inbox getting this log:(Permission denied Please help me how to solve. I reinstalled 3 times but problem not solved (var/log/mail.log)

root@nandini:/var/log# tail mail.log
Jun 12 13:13:26 nandini dovecot: IMAP(admin.kailashvision): mkdir(/var/lib/dovecot/index/admin.kailashvision/.INBOX) failed: Permission denied
Jun 12 13:13:26 nandini dovecot: IMAP(admin.kailashvision): mkdir(/var/lib/dovecot/control/admin.kailashvision/.INBOX) failed: Permission denied
Jun 12 13:13:26 nandini dovecot: IMAP(admin.kailashvision): mkdir(/var/lib/dovecot/index/admin.kailashvision/.INBOX) failed: Permission denied
Jun 12 13:13:26 nandini dovecot: IMAP(admin.kailashvision): mkdir(/var/lib/dovecot/control/admin.kailashvision/.INBOX) failed: Permission denied
Jun 12 13:13:26 nandini dovecot: IMAP(admin.kailashvision): mkdir(/var/lib/dovecot/index/admin.kailashvision/.INBOX) failed: Permission denied
Jun 12 13:13:26 nandini dovecot: IMAP(admin.kailashvision): mkdir(/var/lib/dovecot/control/admin.kailashvision/.INBOX) failed: Permission denied

Status: 
Closed (fixed)