Update, 5:50am - We received word from our vendor that the fix has been implemented. We are seeing normal delivery of new messages. Messages that had been queued during this period are now being delivered and it might take another 30 minutes to deliver the entire backlog.
8/21/2014, 5:00am - Starting at approximately 4:00am both Exchange and Zimbra are experiencing up to a 90 minutes delay on mail delivery of incoming mail due to problems being experienced by our mail sanitation vendor. We have received notification from Message Labs that they have discovered the problem and are working on a solution. They predict that the implementation of the solution should take about one hour.