I had a config working for quite a while using the "Trash" & "Sent" folders (both folders capitalized) on an IMAP server. A couple of weeks ago (probably when upgrading to 0.9.12) I noticed that sylpheed uses the "trash" & "sent" folders (all in lower case instead). Both the GUI & sylpheedrc files display the rightname (#imap/IMAP/INBOX/Trash for instance) but looking in the log when a message is deleted I find the following message: [22:12:33] IMAP4> 209 UID COPY 8081 INBOX.trash.
I don't think it's a Claws bug...
Not a bug. The special folders ('inbox', 'sent', 'drafts', and 'queue') just have an internationalised name in the GUI, which happens to be capitalized.