Yeah, yeah before you start I know there are a million blog entries out there saying this but, they don’t really. With Exchange Server 2010 MaxSessionsPerUser doesn’t exist in microsoft.exchange.addressbook.service.exe.config
This KB clarifies this .. the important bit is “must be added manually”
Note: Under Microsoft Exchange 2010, SP1, this value is no longer included in the microsoft.exchange.addressbook.service.exe.config file. The MaxSessionsPerUser (i.e. MaxSessionsPerUser=100000) value must be manually added to the file and set to the desired number of connections.