Add the Exchangeorganizationdomain to Office365.The migration service uses the SMTP address of the onpremisesmailboxes to create email addresses for the new cloudbasedmailboxes.
Verify that user mailboxes are not hidden in onpremisesaddresslists. Migration will fail for onpremisesmailboxes that are hidden from addresslists.
Disable Unified Messaging.If mailboxes are enabled for Unified Messaging (UM), disable UM before the migration. You can then enable UM on the mailboxes after the migration is complete.