Hi!
I switched over from Thunderbird to KMail (KDE SC 4.9.1) as it is much better integrated with the whole system.
But there is one single feature I really miss from Thunderbird. I use IMAP and filter the mails on the server side to subfolders of the inbox for various criterias (mostly by sender or receiver).
I get instant notification with KMail for unfiltered mails that go straight to the inbox. But if the server puts an incoming mail into the folder "INBOX/KDE-Forum" for example, I won't get instant notification, instead I'll have to wait for the next fetch interval to see if new mail arrived. Thunderbird used the IMAP Idle feature to also get notifications on subfolders.
Is there a way to achieve this with KMail, too?

I have exactly the same problem. Did anyone find a solution to this problem? Btw. I am using dovecot 2.1.7 and it works for the INBOX but not for any subfolder

So far IDLE is supported only for INBOX type folders, IIRC.

Thanks for the reply! Is there any chance to change this? Or would this require very much effort? Is there a specific reason for only supporting INBOX?

I don't think there was any specific reason. I think simply put no one has had the time to implement IDLE support for other IMAP folders.