Server NC027 (the new mail-only server) experienced a great and very brief increase in activity at about 04:00 UTC on 23 January, and we suspect that this lead to the server getting behind in scanning and processing email. The hard drives simply couldn’t keep up.
In order to allow the server to catch up, we disabled virus and spam scanning. Over the course of the coming week we’ll be re-enabling each of these (inbound and outbound) one at a time per day. This will allow us to determine which activity is the bottleneck and make finer adjustments accordingly. We also have a back-up plan to move the accounts on this server to one with faster (solid-state) drives, if necessary.
We apologise again for the inconvenience of significantly delayed email on Thursday. If you have any questions about this incident, please contact NinerNet support. Thank-you.