error: Unknown setting: mailbox in /etc/dovecot/conf.d/15-mailboxes.conf

[chihoko]

Commit c8bd86b creates the file /etc/dovecot/conf.d/15-mailboxes.conf which causes Dovecot to fail on start with the error

Fatal: Error in configuration file /etc/dovecot/conf.d/15-mailboxes.conf line 19: Unknown setting: mailbox