If you don't know the users' passwords , you'll need to reset them to passwords that you do know, and then enter those passwords ...

 If you don't know the users' passwords, you'll need to reset them to passwords that you do know, and then enter those passwords in the migration file. This is inconvenient for users, but there's no way around it unless your source email system supports using your admin credentials. If you want users to have access to the source email system, you can distribute new passwords to the source email system after the migration is finished.