Version: 1.9.1 (using KDE KDE 3.5.2)
Installed from: Debian testing/unstable Packages
Deleting an IMAP message is implemented in 2 steps:
1) copy to Trash
2) mark original as deleted
KMail does not mark the original IMAP message as read (seen). Hence mail notifiers such as Gnubiff, Korn, or even KMail's own mail notification count these messages still as new.
When deleting a message, Thunderbird and Evolution mark it as seen. This way mail notifiers will not include deleted messages in the new message count.
This has been reported before (72424 + 65418). Both bugs have been closed as a duplicate of bug 26986, which they are not; the latter requests two functions: "delete" and "mark for deletion".
From a user point of view, the KMail behaviour is not comprehensible, and it renders mail notifiers useless. Or people resort to compressing the inbox manually as a fudge (http://lists.kde.org/?l=kde&m=116257270628700&w=2).
When I get 100 spam messages, I have *seen* enough of them to delete them and don't want to have to manually expunge them, too. (The "Auto-compact folders" does not seem to take effect until exit.)
*** Bug 160632 has been marked as a duplicate of this bug. ***
*** This bug has been marked as a duplicate of bug 117717 ***