thread pool exhausted (IMAP)

[rgiapratama]

I got some error like this in mailbox.log :

2011-07-05 22:23:02,225 WARN  [ImapSSLServer] [] TcpServer/993 - Thread pool is 105% utilized.  21 out of 20 threads in use.
2011-07-05 22:23:02,225 ERROR [ImapSSLServer] [] TcpServer/993 - cannot handle connection; thread pool exhausted
java.util.concurrent.RejectedExecutionException
        at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:1768)
        at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:767)
        at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:658)
        at com.zimbra.cs.tcpserver.TcpServer.run(TcpServer.java:202)
        at java.lang.Thread.run(Thread.java:662)

zmprov gs -------- zimbraImapNumThreads

# name ------
zimbraImapNumThreads: 500

result free command :

             total       used       free     shared    buffers     cached
Mem:      10233868    9973108     260760          0     374512    3718400
-/+ buffers/cache:    5880196    4353672
Swap:     12289716        144   12289572

This problem caused Outlook error :

Task 'Synchronizing subscribed folders for ---@----.' reported error (0x800CCC0E) : 'Outlook cannot synchronize subscribed 
folders for ---@----. Error: Cannot connect to the server. If you continue to receive this message, contact your server 
administrator or Internet service provider (ISP).'

How i can resolve this problem.
Thank you.