UPDATE: The hardware failure has been fixed. Incoming mail is being received. It may take some time for all cached messages to be delivered.
We have a hardware failure in one server of the Exchange system, so incoming mail is not being delivered to in boxes. If replacement parts are available, we should have them this afternoon. Incoming mail is being spooled for delivery once the server is back online, so no mail should be lost or bounced yet. Outgoing mail appears to be unaffected.