Your company has a Microsoft Exchange Server 2003 organization.
Users access their mailbox by using RPC over HTTP and Exchange ActiveSync.
You purchase an Office 365 subscription. From the Office 365 portal, you create and verify the accepted domain of the company. From the Exchange Control Panel, you attempt to migrate all of the mailboxes to Microsoft Exchange Online and you receive an error message indicating that the remote server is unavailable.
You need to identify what prevents the mailboxes from migrating.
Which tool should you use?