Due to mistake on our behalf, we forgot to install the latest SSL certificate we had for mail.messagingengine.com. The one that was installed expired on Nov 17 05:39:21 2007 GMT. About 2.5 hours after that, we noticed and updated the certificate with the latest one.
During the time the certificate was expired, there were no security issues using it, it just meant that most email software would have popped up an error dialog telling you that the certificate had expired, which is annoying, but it still left email accessible after accepting the certificate.
To avoid this happening in the future we're going to setup our regularly checks process to retrieve the currently installed certificate and check it's expiry date. If it's less than 2 weeks in the future, we'll make sure we're emailed so we don't forget to deal with it there and then.