Exchange Migrator 2.x
How does Exchange Migrator count licenses?
How does the Exchange Migrator license model work?
Exchange Migrator subtracts 1 license from the license count for each Mailbox you migrate and adds 1 license back to the license count for each Mailbox you unmigrate (undo migration).
Migrating or unmigrating Recipients, Distribution Lists, or Public Folders does not affect the license count.
Important: If you migrate a Mailbox and then delete it on the target server, Exchange Migrator cannot add the license back to the license count. Be sure to unmigrate unwanted target Mailboxes to replenish your Exchange Migrator license count.