We are currently addressing a processing delay in ExchangeDefender antivirus scanning engine. One of our virus engine vendors had distributed a faulty update which has caused a backlog of messages that have been quarantined for further inspection.
The ExchangeDefender system is designed to apply stronger scrutiny and more intensive checks against any attachments or messages that have produced any sort of an error in any of our antivirus scans. The reason we run multiple scanner engines is because not all engines are as thorough or as rapidly updated as the threats emerge and change in the wild. Once an issue is encountered we scan with more options to find out if the message is indeed dangerous or if there is something wrong with a portion of it (attachment, envelope).
In this case the corrupt message was passed on to ExchangeDefender which quarantined messages for further scanning which is far more expensive and processor intensive. We have responded immediately and removed the engine, however, even slight issues can cause huge problems when you process as much mail as we do and it has introduced a slight delay in the processing of messages. The issue started at roughly 3:10 and was resolved by 3:40. At the time of this message we see around 60% of our nodes processing messages within our ordinary SLA (seconds) and we expect the rest of the network to catch up shortly.
If you experience any delays, even extensive in nature, it is due to the above problem which will within 30 minutes be completely under control.