Keeping multiple OAB mailboxes in same AD site , is always efficient Method of OAB distribution. Because I know Some AD sites have large number of user mailboxes deployed. Instead of recommending to deploy single OAB mailbox, can we think of an architecture where, OAB multiple OAB mailboxes keeps track of users, when they download OAB. Something like a checkpoint. ( few bytes of info) and Sync the Checkpoint across all OAB mailboxes. Discard the checkpoint once in 24 hrs or 48hrs. Checkpoint can be an indication that user has already downloaded the OAB .. Ignore another download.
only exception when user creates new profile / multiple profiles / manual full download.
Nat.