I am running a 2010 Exchange server in multi-tenant mode and have two users who have the same name in 2 tenants.
When I am logging in via OWA everything looks fine.
As soon as I add one of the users via Outlook and open the mail is correct however the GAL is either correct or the other user that happens to have the same name GAL!
I created a generic account in AD (not via OWA management) so it was completely unrelated to both and tried to add each Outlook profile but this time it was pulling down a blank GAL (authenticating as the new user with nothing?). I have got about 150 users over 10 tenants on the server and never had a problem!