I've picked up a couple of these automated messages today in our ticketing:
"Unable to deliver message to randomuser1@domain.com. Error message: An existing connection was forcibly closed by the remote host."
Unable to deliver message to randomuser2@domain.com. Error message: Timeout::Error
Also today got an error from another user on his smartphone:
"Microsoft.Exchange.Clients.Security.LiveTransientHRESULTException: LiveId authentication code has returned error 0x80049234 PP_E_RPS_REASON_POST_TICKET_TIMEWINDOW_EXPIRED,indicating that there's a temporary problem with the remote server. Please check the Application event log for detailed information and retry the operation later. --- System.Runtime.InteropServices.COMException (0x80049234): Post ticket time window expired."
I'm guessing it's an Office365 latency issue, as all these mailboxes...