The optimization of a EAS synchronization process is that a client's sync key is not updated if no data was indicated as changed during synchronization step. This behavior is not strictly compliant to EAS protocol specification. Thus this behavior is made configurable through option 'com.openexchange.usm.force_new_sync_key_on_empty_ sync' in file 'usm.properties'. By default this option is disabled (set to false); meaning described behavior is activated.