what is the "stage." directory for?

[Marten Lehmann]

Hello,

in my setup with cyrus 2.2.12 I have a directory named 
"<partition-default>/stage.". It includes files like

8801-1161887510-0
8802-1161887656-0
8807-1161887123-0
8808-1161887204-0
8809-1161887238-0
8810-1161887139-0
8811-1161887596-0
8812-1161887667-0
8838-1173199589-0
8852-1161887124-0
8925-1161898745-0
9073-1161684396-0
9078-1161684092-0
9086-1161684072-0
909-1162808438-0
9097-1161684063-0
9104-1161684064-0
9109-1161685066-0
9120-1161685680-0
913-1162810100-0
9372-1161685682-1
945-1164642075-0
946-116281075966-1164642072-0

which contain complete messages including headers. Most of them are 
several months old, but there are also files from today. What are they 
used for? Is it safe to delete them (shutting down cyrus first)?

Regards
Marten Lehmann