In some scenarios it's not possible to run an address book sync against the mail system. This could be for various reasons, such as: it's a hosted environment to which you don't have PowerShell access, it doesn't exist (museum view only), or it hosts vastly more users than you need to target. In these cases it's possible to have the SyncAB run against a file. The file must be in XML format and follow a specific schema. Find a minimum-spec example for Exchange illustrated below.