Some reports came in as mail being returned. Tranced to BT's DNS servers baing down, thereby affect anyone using them directly or in transit. Issue was resolved by them fairly quickly.
One smtp cluster, core4, reported an invalid SSL certificate to users. This was due to last years pem certificates being loaded on boot in error. There was no impact to mail delivery for any customer, however, in some instances a warning appeared on a clients screen advising of invalidity and asking to proceed. After the correct pem SSL file was added, the warning has gone away. This was a low priority warning and considered advice only.
Below is a real-time overview of our servers where you can check if there's any known issues.