You normally don't have to do anything on the concentrator/client configs. If you are connecting the client, are you able to ping the Exchange server? Try reducing the client mtu to say 1400 with the set MTU utility and see if it makes a difference.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...