Our engineers have been working through the limited number of reports of the issue and believe this relates to the outbound mail service we use for some mail to certain destinations to aid with delivery.
Logs indicate messages to Gmail, Outlook (including Hotmail/Live/365) and IONOS are affected.
Outbound messages to destinations which have shown a higher error rate have been redirected and we are in the process of identifying any tickets affected by this and will send a further update.