Mail Users are similar to Contacts in that they do not have a mailbox in the local Exchange organization, however unlike Contacts they do have a user account in Active Directory.
In other words, Mail Users are mail-enabled user objects that use an external email service.
I go back to the Outlook connectivity test feature, click "Connection Status", and click "Reconnect". I turn on Cached Exchange mode and when I open the address book this time, the GAL is populated. I tried to search for a name that I knew was in the address book Strangely, although my GAL is empty, there is a result. CN=[user name], right click, then select "Properties" 4.
Now when I open the address book, the GAL is populated! I close the address book and open again, and it stays populated. However, when I turn off Cached Exchange mode, the GAL goes back to empty mode again. I've been working on this for days now, so I decided to just keep Cached Exchange mode on since it fixed the problem. I double click on the result Click Member of and if you have something like ALL Users, add that to the Contacts. In the Attribute Editor tab, find ms Exch Query Base DN attribute 5.
Then suddenly, when cached mode was on, the GAL was populated!