Thanks, I found the solution. The name of the bad mailbox wasn't in fact empty, but a single blank character which the cyradm and quota commands dutifully removed as "leading blank" in their output. So
localhost> sam "user/ " cyrus all localhost> dm "user/ "
was what it took to get rid of the mailbox with the "empty" name, and
localhost> sam "user/ 10000" cyrus all localhost> dm "user/ 10000"
took care of the second one created by the hapless colleague's setquota command.
Am 07.05.2013 21:04, schrieb Tilman Schmidt:
A junior administrator of mine mistakenly created a mailbox with an empty name, by typing into cyradm the commands:
createmailbox user/$lb setquota user/$lb 10000
[...]