what is the DELETED directory in spool

[Eric Doutreleau]

i m using cyrus 2.3.16
and in each spool on the same level as the user directory i have a 
directory whose name is DELETED.
i am wondering what is this directory?
it contains a mailbox hierarchy with some mails which are very old
can i smoothly delete it?
----
Cyrus Home Page: http://cyrusimap.web.cmu.edu/
Cyrus Wiki/FAQ: http://cyrusimap.web.cmu.edu/twiki
List Archives/Info: http://asg.web.cmu.edu/cyrus/mailing-list.html