Seperate GAL's based on OU for remote offices?

I have an interesting setup that a client of mine is wanting.  They have approximately 30 different offices and each office cannot see the other offices users.  This is how they are wanting it to be setup within Exchange...  

There is only 1 Exchange 2010 server.  Only the HQ office is on the domain.  The remote offices are not on the domain so there are no additional AD sites to separate them.

Each office is a separate OU and their users are within it.

Users within specific OU's would also have the ability to share calendars, contacts, and tasks with each other.

Is it possible to setup 30 different address lists that only include the users within each particular OU's to have access to?  No GAL with all users within it as each office cannot see anyone but their own office contacts.

Curious if anyone has done something similar in the past and looking for pointers.

Thanks for any help!
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You can refer GAL Segmentation \ Segregation  feature of Exchange 2010 for same. Please refer below link for same.

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