hmmm ... so maybe use Full Control or Custom Control in ACL rather than just Read/Write. Read/Write should do it but ... if it's not working then another option? I used Full Control (not sure of the exact terms now since I'm at home) but I know that folders created do inherit parent folder...
You should try http://www.entourage.mvps.org/ It's one of the best I've seen. I fixed a similar problem a long time ago, and I think it was under the Advanced server settings. Like using the same settings as the receiving mail server? Good luck!
Start Terminal and type (or copy/paste from here, 1 line at a time followed by carriage return):
cd /System/Library/Frameworks/JavaVM.framework/Versions/1.4.2/Libraries/ (return)
sudo ../Commands/java -Xdump (return)
The sudo might take a little while, but when you get back to the prompt...