Here is a brief overview of how our system is currently configured (which I rather like).
We have a Samba and OpenLDAP domain controller that handles computer logins. We have modified the LDAP directory to contain the additional information needed by Scalix. (This information is found in the scalix schema files). We have also modified the smbldap tools useradd script so that when we create a new user for the domain it will automatically create the necessary Scalix options. (Like mailbox size limits, admin priveledges, etc.) We use phpLDAPAdmin for simple administration of the directory. (Like adding additional email addresses, name changes, etc.)
Our Scalix server is version 10.1 running on FC4. We use omldapsync in a cron job that runs every 30 minutes. Omldapsync takes care of user modifications that we make on our OpenLDAP/Samba system, and will add new users, and delete them as well. If I go into SAC I cannot manage any of my users here. They are all greyed out, this is becuase I have chosen to manage them from my current LDAP diretory and merely 'sync' them together. Users that I do not need in samba I create in SAC. This includes email users like support @domain.com, etc. For distribution lists we use GNU Mailman. See other posts by me for information on mailman.
Hope this helps you see the big picture of our setup so it can help you with yours.