Are Personal Folder Files (.PST) being used as a temporary repository when migrating data?
Exchange Migrator 1.x
Exchange Migrator 2.x
Exchange Migrator (EM) uses a technology that efficiently streams the data so there is no need for a temporary repository. When EM is installed on the target machine, the data is streamed directly from source to target Exchange server. If EM is installed on a console machine (another machine that is not the source or target machine), the data is streamed from the source server, through the console, to the target Exchange server.
This information can be located in Chapter 3 of the Exchange Migrator (EM) User Guide in the sections titled 'Exchange Migrator Configurations' and 'Understanding Data Transfer'.