[Exim] folder ownership

Hi,

I'm hosting a few domains, and I want to give the domain owners
control over their own virtual alias file... I would LOVE to have them
able to create their own email address prefixes and folders (since I
don't want to add an entry in the password file for every folder).
The only problem that I have is that they are all owned by "exim" or
whatever I have listed as the default user.

I would like to have the following scenario:

alias-bla.com:
username1: username1
username2: username2
username1-spam: /home/username1/Mail/spam
username2-spam: /home/username2/Mail/spam

Is there any way I can have /home/username1/Mail/spam owned by username1
and /home/username2/Mail/spam owned by username2? I can manually set it,
but exim changes ownership back to exim and puts 600 permissions on it.

I have figured out how to chnage the owner for the entire domain:

trusted_users = amavis:smmsp:username1
...
  virtual_bla_com:
    ...
    domains = bla.com
    user = username1

I'm wondering how to get multiple usernames within a domain.

Thanks!

-Mike

--
Michael J. Weller, M.Sc.
University of Windsor
Windsor, ON, Canada