The outage causing customers to be unable to reach email has been resolved. We're investigating the cause, but we have also recently been building a new mail system to replace the existing infrastructure. It's almost like the old system knows it's going to be replaced soon!
A security certificate on one of our back end systems had renewed a little while ago (as expected), however, some systems had not reloaded this new certificate and were using the previous certificate which expired today. Because the certificate that was in use had expired, the system refused to connect to the mail store. We will be reviewing how renewed certificates are reloaded by these systems.