Well, generally speaking, yes ... AD is the prefferred directory service. And in my personal experience I have setup BES express with Scalix connector in two different instances always with AD available.
This said there are a few consideration to underline.
- The BES version compatible with Scalix Connector is the one for Exchange ... and this latter does not work without AD
- We must separate two functionalities of BES: mailbox scanning and sync from authentication. Former is mainly related to GAL (which is provided by Scalix Connector itself) while the latter has, at present, the only option to authenticate in basic mode or against AD
- I've seen many log entries from BES trying to resolve SMTP addresses accessing AD
Hope this makes any sense to you and leads you in the right direction.