I'm coming in a bit late but I've checked the archive - I had this problem and I solved it by changing the permissions of the socket file to cyrus.mail and also the permissions of the deliver program to cyrus.mail with sticky bits set for user and group execute. File ran and delivery was successful...
chmod /usr/cyrus/bin/deliver g+s chmod /usr/cyrus/bin/deliver u+s Hope this helps, Rich.
