Extension to the maildir[1]-inpired directory structure.

Errored jobs. Open an https://Seppo.mro.name/issue or
ask in the https://Seppo.Social/mailinglist. Purge manually.

[1]: https://web.archive.org/web/19971012032244/http://www.qmail.org/qmail-manual-html/man5/maildir.html
