Version:           1.9.5 (using KDE KDE 3.5.4)
Installed from:    Ubuntu Packages

When a folder called "Name" is created in maildir format within Kmail, a directory called "Name" is created, together with auxiliary files ".Name.index", ".Name.index.sorted" and ".Name.index.ids".  When this folder is deleted from within Kmail, the file ".Name.index.ids" remains.

The left-over file does not seem to be necessary, for it can be deleted manually without any apparent ill-effects.