Courier IMAP - Outlook 2003

["Serhan D. KIYMAZ" <serhan@kiymaz.org>]

Hi,
A few weeks ago I've installed qmail, courier-imap, vpopmail, tcpserver 
for my company to send/receive emails.
Everything is fine with Kmail and Thunderbid. It's possible to 
send/receive mails, reach virtual shared folders.
But when Outlook 2003 and Outlook Express clients want to check mail, 
they get following error:
"A connection to the server can not be established. mail.xxx.com.tr is now
offline mode"
Anyone got an idea about this problem? How can I fix this?
Is this an Outlook oriented problem or is there something wrong about my 
installation?
Regards