So I thought I’d try Evolution, the Gnome email / calendar / messaging Borganism, which can allegedly support maildir files. Installation went swimmingly, but (at least at first glance) it can’t. More on this later.
Evolution uses the Gnome keyring to store passwords, which is (of course) different from and incompatible with the KDE keyring. The principle seems to be that when you sign on / log in / whatever, the Gnome keyring’s login keyring automagically unlocks, giving Evolution access to the POP3 password used for your ISP. Except that didn’t work; Evolution uses the default keyring, not the login keyring.
Evolution / Gnome evidently creates a default keyring entry by scraping up a password from somewhere unknown to me. The only way to clear it, at least in XFCE, without installing the entire mumble Gnome desktop, is to simply delete the keyring file… at which point, the keyring manager will ask you for a password and recreate the file.