[Dovecot] Root folder path problem

I use dovecot1.x. for imap on qmail. I also use OExpress to receive my mails. My sent mails is stored in main "sent items" of Outlook Express. Therefore I set "INBOX" in "Root folder path" option. But when I send an email to anywhere the mail sends without problem but I am getting an error as following;

Some of the messages in your Outbox Folder could not be copied to your Sent Items.

How can I correct that ?

Thanks