When migrating to Office 365/Exchange 2010 the following error is seen:-
Migration failed: InvalidUser
This error indicates that the specified user does not exist in the target Exchange system, and auto discovery failed. You should ensure that the combination of ‘Import Name’@’Domain Name’ matches the primary SMTP address of the users in the target system. If the tool is being used to create users (using PowerShell scripts), you should check that the scripts are correct and are being executed as part of the migration process.
This can also happen if the user mailbox has not yet been fully provisioned. This can occur shortly after user creation before all the backend processes have been completed. If you are sure the users exists and are getting this error, wait a while and try the migration again