have you used finally Exchange to verify valid users?
MEG 7.x by default will update based on its synchronization task. You can change the schedule to be more frequent if needed or change the configuration to be performed in realtime lookups.
Realtime lookups are usually a bit more on the DC for LDAP lookups from MEG however.
You can also refer to the following Blog Posted we did for LDAP.
If we uncheck the option for Cache Results the appliance will perform the realtime lookups.
hello roadwarrior. I ended up just sticking with LDAP. It is very straighforward and more widely supported than trying to pull from Exchange.