Marc,
There are two things that you can do to get this information:
1. Use MBINFO. I know you mention that "it does not work" but - we have used MBINFO against an Exchange 2003 server in Support, this should work. You do need to make sure that you have permissions, like when you are running Exmerge, however it should work.
2. Use the PFDAVAMIN tool. When connecting, use the "Connect to mailboxes" option. If you have permissions, connect to "all mailboxes" and then go to Tools > Export Permissions. (logging will need to be turned on under Tools > Options). You can grab PFDAVADMIN
here:
http://www.microsoft.com/downloads/details.aspx?FamilyId=635BE792-D8AD-49E3-ADA4-E2422C0AB424&displaylang=en