It was a UKFSN problem - not an Enta problem.
Our email filtering server uses both spamassassin and dspam to handle classification of spam emails (with a number of additional plugins) and dspam had managed to get into a blocking problem which I had not anticipated. As I was away from any form of electronic communication until quite late on Saturday I didn't discover this as quickly as I normally would.
I managed to resolve the problem just after midnight and confirm that all email had been delivered by 00:30.
On Sunday morning I reconfigured dspam so that this problem cannot happen again.