kmail2 updates mail status to late

Hallo

one thing I found recently: If I read a mail with kmail2 the read status was 
not set on the server in time. About five minutes after I read a mail I 
started thunderbird and the mail was still marked as unread.

In what time the status update should be set onto the server? With online imap 
I assumed this would happen almost immediately.

I use online imap with cyrus imap server. 

Regards

Martin
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