The root cause was an incident on the SITA network that has been acknowledged in the following notification.
Unexpected Message Delay – SMTP and MSW Hosts Type B & MQ Services - Open/Close - INC0695908*
_From: Messaging.Event.Advisory@sita.aero_
Unexpected Message Delay Affected: Unexpected Message Delay – SMTP and MSW Hosts Type B & MQ Services Details: Please be informed that we observed an unexpected message delay for our SMTP and MSW Hosts Type B & MQ Services. As of 2019-08-14 21:10 GMT, the issue was resolved. During this time, we noticed a backlog which caused delay in message delivery. All Services are back to normal now.
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Ink has implemented improvements on our own infrastructure to be able to better handle this type of interruption.