rwx permission to mailbox files

[Alexis Perrier]

Hi
I am totally new to postfix
I am using postfix / mysql / courier and would like my application
(ruby on rails) to be able to read / write or execute the incoming
local mailbox files.

Each time a new email addressed to users...@domain.com arrives, it
is stored in a folder
/home/vmail/domain.com/user.name/new

Right now the permissions for the mail files in that mailbox folder
are restricted to the vmail user and group I used to set up postfix :
for instance :
-rw------- 1 vmail www-data 24231 2009-06-15 12:25
1245068734.V801Ie1985M179995.qameha

I am trying to find the correct parameter in /etc/postfix/main.cf so
that the permission on this file would be something like
-rwxrws--- 1 vmail www-data 24231 2009-06-15 12:25
1245068734.V801Ie1985M179995.qameha

I tried many things already but cannot find the right set of
parameters.

Thank you for your help
Alexis