Problem with cyrus-imapd <-> outlook

[Jose Antonio Becerra Permuy]

Hi everybody!
I have a very strange problem and I write this to see if somebody else have 
face it before.
The imap server is cyrus-imapd 2.1.14.
Clients are Microsoft Outlook XP running on Windows XP.
Four clients have no problems at all.
One client hangs many times (70% more or less) when trying to access to the 
imap server. It only happens in this machine. It happens with every user. The 
same users can use outlook in the other machines without problems. Outlook 
Express works well in this machine (but the user wants Outlook XP). I already 
have tried the usual and desperated method of formating and reinstalling 
everything in the windows machine without success.
I have captured the network traffic between client and server with tcpdump.
Failed connection (I have hidden domain, user and password):

* OK www.domain.com Cyrus IMAP4 v2.1.10000 CAPABILITY.
* CAPABILITY IMAP4 IMAP4rev1 ACL QUOTA L0001 LOGIN "xxxxx" "yyyyy".
0001 OK User logged in.
0002 IDLE.
+ idling.
DONE.
0002 OK Completed.
0004 IDLE.
+ idling.

Successfull connection:

* OK www.domain.com Cyrus IMAP4 v2.1.10000 CAPABILITY.
* CAPABILITY IMAP4 IMAP4rev1 ACL QUOTA L0001 LOGIN "xxxxx" "yyyyy".
0001 OK User logged in.
0002 IDLE.
+ idling.
DONE.
0002 OK Completed.
0003 LIST "" "".
* LIST (\Noselect) "." "".
0003 OK Compl0004 LIST "" "INBOX".
* LIST (\HasNoChildren) "." "INBOX".
0000005 LSUB "" "*".
* LSUB () "." "INBOX".
0005 OK Completed0006 IDLE.
+ idling.
DONE.
0006 OK Completed.
0008 IDLE.
+ idling.
DONE.
ZZZZ LOGOUT.
0008 OK Completed.
* BYE LOGOUT received

It seems that outlook sometimes doesn't send LIST command (?!). I am 
absolutely lost. If somebody can help me I will be very grateful.
Regards.