This just helped me out, so I thought it was worth noting
Error message: Your mailbox appears to be unavailable. Try to access it again in 10 seconds. If you see this error again, contact your helpdesk.
Test-ServiceHealth | Select ServicesNotRunning | foreach { Start-Service $_.ServicesNotRunning }
https://social.technet.microsoft.com/Forums/exchange/en-US/d0977cb3-646b-44ca-b30b-34d2abe58a6e/exchange-mailboxes-unavailable?forum=exchange2010
