IMAP IDLE in kmail doesn't work ?

imap idle is enabled in my gmail account.
Kontact - Wikipedia, the free encyclopedia says kmail support imap idle.
i configured my imap gmail account, enabled “启用服务器端订阅” which i guess means ‘enable imap idle’ in the advanced tab, and disabled periodically check email in the regular tab.
but when i have sent a mail to this account from another email server, kmail seems not receive this email instantly. i have to check email manually.
what’s wrong?