I have spent some time searching around trying to find out how I can modify the list that appears whe my users click on the "to" button. The list that now appears consists of both user mailboxes and room mailboxes, and I would of course try to just show the user mailboxes. IN addition, working at a school, we are looking at adding user mailboxes for all the students as well but say when the teachers and staff click on "to", they will not see the students, but rather the staff and that Students will be organized and selected from the drop downs. However, I just can't seem to get my head around how this is supposed to be done, any tips?

This article should answer your questions. Take note to the bottom section, discussing Custom Address Lists. Understanding Address Lists http://technet.microsoft.com/en-us/library/bb232119.aspx

you need to use multiple address lists and permission them appropriately. If memory serves, you'll need to remove permission to the default GAL for everyone (maybe even deny), create seperate GAL's for each definition with the AD search strings you want, and then do the same with OAB's (assuming your using outlook cached exchange). Each GAL/OAB will have to be permissioned appropriately as well.

Thing is, I have done all this, I have create new address lists using OPATH and filters and what not, what I cannot understand is how to modify the GAL?I have tried running the set-address list command in order to change the GAL; but MS is not very fond of this, I have read people using adsiedit to work around this, but this is hardly "best practice" I think.I feel all alone here, I really cannot understand that I am the only one running Exchange 2007 wanting to replace the GAL with a custom GAL/address list, what do you guys do?I appreciate the effort and links and tips you two have provided but I still cannot get this into my head...

Did you deny accessfor requiredusersto Default Global Address List? It should hide it from Outlook.